The Most-Cited Domains in AI: A 3-Month Study

In September 2025, ChatGPT suddenly began citing Reddit and Wikipedia—two of its most frequent sources—far less frequently.

SEOs were quick to point out a correlation between the decline and a change in how Google serves search results. But is that really the cause?

We analyzed prompt data across LLMs to see exactly what happened. But before we get into the research, let’s explore what many SEOs think happened and why that might not be the full story.

The Widely Discussed Theory Behind Reddit and Wikipedia Citation Declines

Google removed its num=100 search parameter in mid-September, limiting various tools’ abilities to access URLs beyond the first page of results. 

If ChatGPT relied on the num=100 parameter to check deeper rankings that contain Reddit and Wikipedia results, this could explain the sudden drop. Something Lily Ray, VP of SEO Strategy & Research at Amsive, pointed out on X:

I asked Sergei Rogulin, Semrush’s Head of Organic and AI Visibility, for his thoughts. 

Personally, I don’t think [the num=100 parameter removal] is the root cause. Or at least, not the only one. Correlation doesn’t always imply causation.

Sergei Rogulin, Head of Organic and AI Visibility at Semrush

Sergei points to data from Semrush’s Organic Rankings tool as support. We can see that out of the ~263M keywords Reddit ranks for in Google, only about 90M (34%) of those rankings are between positions 21 and 100. 

That doesn’t align with how dramatic of a drop we saw in Reddit citations from ChatGPT, something Sergei elaborates on: 

I believe the main reason for the drop is an attempt to avoid over-citing on certain websites, to be less biased toward them, while generating answers. As a result, ChatGPT has become more resilient to manipulation attempts.

Sergei Rogulin, Head of Organic and AI Visibility at Semrush

What’s clear is that the event exposed how volatile AI visibility can be. 

Key Takeaways

• Reddit and Linkedin were among the top five most-cited domains on ChatGPT, Google’s AI Mode, and Perplexity
• ChatGPT had the sharpest adjustment among top citation sources, with significant drops for Reddit and Wikipedia occurring in mid-September
• Despite the drop in citation share, Reddit and Wikipedia are still ChatGPT’s two most-cited domains
• AI Mode has a clear preference for Google’s owned domains

Study Methodology 

To look closer at what really happened with AI visibility changes, we checked weekly citations for more than 230,000 prompts over 13 weeks across three LLMs. 

Here are the details:

• We analyzed 230K prompts across ChatGPT search, Google AI Mode, Perplexity
• We took weekly snapshots between July 14, 2025 and October 12, 2025 (thirteen weeks)
• Each week, we studied the top 25 domains that appeared most often among AI citations
• We analyzed over 100M total AI citations

The Most-Cited Domains Across AI Platforms

Despite the dramatic drop-off in ChatGPT citations, Reddit and Wikipedia are still among the top cited domains across the LLMs as of October 2025.

Let’s look closer at some of the top domains’ citation volatility across the AI platforms we studied.

Reddit: Abrupt Decline on ChatGPT, More Stable on AI Mode & Perplexity

ChatGPT cited Reddit in close to 60% of prompt responses in early August before collapsing to around 10% by mid-September. 

Wikipedia: Abrupt Decline on ChatGPT, More Stable on AI Mode & Perplexity

In ChatGPT, Wikipedia dropped from appearing in roughly 55% of AI prompt responses to less than 20%.

In contrast, the portion of LLM responses Wikipedia was cited in on AI Mode and Perplexity stayed consistent, holding near 3% on AI Mode and 0.8% on Perplexity.

This mirrored Reddit’s ChatGPT drop around the same time and shows that there wasn’t a systemic drop across all LLMs.

LinkedIn: Steady Rise Across All Platforms

AI Mode consistently cited LinkedIn in nearly 15% of its responses, dipping briefly in early September but quickly recovering.

ChatGPT and Perplexity cited LinkedIn less often, but also trended upward after the timing of the Reddit/Wikipedia drop.

It’s possible that LinkedIn content gained some of the citations that Reddit and Wikipedia lost. 

Forbes: Steady Rise Across All Platforms

Forbesalso showed a clear growth in the dataset in the same timeline.

In ChatGPT, Forbes doubled its number of AI responses with a citation after September. Forbes’ appearance in AI Mode and Perplexity showed smaller increases around the same time.

Winners and Losers After the September Change

Next, we looked at which of the top sites had the biggest positive or negative changes in how frequently they were cited in responses after Google removed the num=100 parameter (around September 11, 2025).

The biggest winners on ChatGPT? PRnewswire, Forbes, and Medium. 

Along with Reddit and Wikipedia, TechRadar experienced a drop in ChatGPT citations. 

The most movement on AI Mode came from user-generated content (UGC) domains. 

YouTube, Reddit, and Facebook grew the most, while Medium, Quora, and LinkedIn had the biggest declines. 

The biggest positive changes on Perplexity were for Wikipedia, Microsoft, and Forbes—although these bumps were minimal compared to the volatility on ChatGPT and AI Mode. 

The biggest overall shift on Perplexity was the negative trend for Reddit citations. 

The Most-Cited Domains on Specific AI Platforms

We can really see how the Reddit and Wikipedia drop in citations was isolated to ChatGPT with these graphs showing the movement of the top five most-cited domains on each of the three LLMs.

ChatGPT 

Before September, ChatGPT responses cited Wikipedia and Redditabout five times as much as the next most-cited domains. 

After mid-September, the two giants came closer to the others, which include Medium, Forbes, and LinkedIn. 

Here’s a closer look at those five weeks immediately before and after the removal of the Google parameter in September.

Reddit and Wikipedia dropped dramatically before stabilizing and re-establishing their lead in a more balanced ratio. 

Google AI Mode

AI Mode showed a far more balanced mix of cited domains throughout the quarter. Its top cited domains included LinkedIn, YouTube, Reddit, Google, and Google Blog(all of which are properties that Google either owns or has a major partnership with).

While there’s some movement—Reddit citation frequency rises in late August, then falls again—the overall composition at the top remains more stable than for ChatGPT through the study duration.

Compared with ChatGPT, Google’s AI Mode cites Wikipedia much less frequently. Wikipedia was cited in around 2% of AI Mode responses.

Perplexity

Perplexity’s top sources were Reddit, LinkedIn, NIH, Microsoft, and Google.

Reddit’s visibility on this platform also declined slightly in September, but the rest of the major domains remained relatively consistent.

Why The Recent AI Visibility Changes Matter for Marketers

The data doesn’t tell us exactly why these shifts happened—but it does show how quickly AI citation patterns can change. And that those changes aren’t uniform across engines.

For marketers, the takeaway is to start tracking AI visibility the way we already track search engine results pages (SERPs).

When citation patterns shift, it's a signal. Those movements tell you which sources AI systems are learning from and trusting.

What You Should Do Now

This high-level data about AI visibility changes among major platforms is interesting. But the real opportunity is understanding your specific industry and topic area.

Which domains dominate AI citations in your niche? Which platforms are gaining or losing ground? Where does your brand appear—and where are the gaps?

Your goal is to take that insight and turn it into action.

Figure out which domains are trusted in your category. Build relationships with them to strengthen your own brand's authority.

Semrush lets you track your brand visibility on AI platforms and search engines. Plus, you can discover which domains are shaping conversations in your niche to inform future relationships.

Use Semrush to get a look into what’s happening from multiple angles.