George Takei is active of mastodon. There are many journalists and scientists. Tons of developers. A few small celebs. Also, many people I don't want to be here are not here. That's a plus for me.
While not part of the fediverse if you want Twitter like activity I would suggest BlueSky. However I am also on Mastodon and I find it to have plenty of activity. It’s not the fire hose that Twitter is but to me that makes it much more manageable.
I can vouch for Signal. I got my whole family out of facebook messenger to make a new place for family chat. Even my parents in their mid 60s had no problem changing.
I can understand it. I was banned without reason from 3 of the top mastodon instances before even posting anything. Creating new accounts is a hassle, and it's easy to lose faith in the system when bans happen without reason and none of the instances cared to respond to my appeals. In heinsight, I'm sure the ban was due to my username looking like a hash, but I still find it crazy that the appeals were ignored.