I think twitter would be a big one, and maybe reddit would be good to add. Not sure if instagram/facebook would fit but idk.
i'm on twitter, and this week i signed up on vk. i'm also on reddit, i have pretty much the same username everywhere. i hate anonymity and privacy
far too many. i plan to delete a lot of the accounts on more mainstream services (imageboards and textboards are okay nearly always but the bigger the sites get the worse they are to use). maybe have a custom field system or something? it would be really cool to be able to have a list of maybe 10 services and be able to click on the service name to find others on subreply who also use it, though when people use different names for the same service that could get weird
I don't use any other social networks except for this one... I feel like an extraterrestrial. :D
Fediverse (Mastodon, Pleroma, etc), personal website, lobste.rs, Hacker News, Teddit, and some others. I'd say that arbitrary key-value fields on profiles would be better; one column can be the name of the platform while the other an be a URL, similar to what most Fediverse sites do.