Started on Mastodon around the time of Elon announcing he would buy Twitter last spring.
Instantly randomized all account data, archived my content, requested right to be forgotten, and deleted my account. (I don't care about him or his politics, I just didn't think he would run Twitter well, and that proved to be correct).
As for Reddit, like others, Lemmy. I started with KBin after FediTips recommended it, but I can't stand it due to the “OSFA” approach of mixing Twitter/Reddit.
As for YouTube, I don't/can't. Though, I do use my own private Invid/Piped server.
Overall, I can't say I miss any of the mainstream services. Though we've had a massive Redditor influx on Lemmy after the API did shut off for many apps, I still find the content in specific communities, much higher than that of Reddit with its memes, bots, and power hungry mods.
I like that on Lemmy, everything everyone does is public, so you can see if a company is astroturfing or if a person is a troll; while there are privacy concerns (I take privacy extremely seriously with email aliases, phone aliases, VPNs or Proxy when applicable) with having everything public, but as long as you don't use your real name or anything linked backed to you, I'd say you're fine.
By having everything visible, I feel like it makes the community much more open and transparent than that of Reddit, Twitter, etc.