Don't read this if you have issues with FOMO. 😉 I've been doing some investigation into hashtag federation (using #activitypub as an initial test tag). I'm seeing mastodon.social has the best tag coverage (not surprising, since it's the largest Fediverse instance). The next largest Mastodon instances (by MAU) have about 90% of the posts. I sampled some instances with around 500 MAU and they had about 50% coverage. The #TagsPub service announces 12% (30 day) to 15% (7 day) of them.
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Most, if not all, of these instances use at least one relay. However, relay effectiveness varies greatly (depends on connected instances).