There used to be the Alexa ranking for websites but it seems to no exist anymore. Elsewhere I only could find for lemmy.world ranking (something like 55000, yay!) but not the actual userbase, and nothing about any other instance (let alone the entire Fediverse).
I think significant media outlet uptake is more of a Mastodon metric, Reddit has a few high profile media outlet accounts - Tampa Bay Times I remember seeing quite a bit, they did a lot of good journalism about Scientology because Clearwater, but I think a lot of news companies slept on Reddit while it was worth using.
I think we have hit an interesting level for notability though. I use Bazzite Linux, and last month they sent a broken update for their version with legacy Nvidia drivers. I was one of the first people on the internet reporting having problems in a post here on Lemmy. As I stated to search my problem, I found that someone on the Bazzite forums linked to my Lemmy post because they had the same problem. They never commented in my thread and they may not even have an account. When’s the last time you saw a Tildes link in the wild?
I think this is the key to natural growth. Especially as more mainstream sites move to login required, and more and more search results are AI slop. Lemmy could be the last open and search indexable platforms left soon