They’re spread all around. The ‘big’ lemmy communities have 2-5,000 monthly users, which probably means a few hundred to a thousand daily users. In the more active communities I follow c/[email protected] or c/[email protected] I’ll see a half dozen or so posts a day and up to 50-ish comments on a super popular topic. Most of them get just a handful of replies. That feels about right to me: the vast majority of people are lurkers, and the vast majority of accounts are abandoned.
It’s why the commercial sites fought so hard for market share and why being The site for microblogging/link aggregation/image sharing is so important. People go to those sites because everyone is there, and everyone is there because that’s just where you go.
There’s no lemmy-wide algorithm making sure you have shiny new topics to look at. The lemmy “all” page is not at all equivalent to r/all, especially on an instance as small as fediverse.boo. The “All” tab is only going to have content from communities that at least one person on the instance has subscribed to, and with only 6 active users ( fedidb.org/network/instance/fediverse.boo ) that’s not likely to be a large set. It’s also possible that the federation mechanics result in you seeing less or delayed content from other instances. Maybe try browsing, even without an account, lemmy.world or sh.itjust.works