Can't say for most people, but I can share my experience.
I signed up for Lemmy and Mastodon around the time the Reddit API news broke out. Before that, I use Reddit for news and discussions in communities I'm interested in, and Twitter for following internet people. For now, Lemmy has been satisfactory enough for me to not want to go back to Reddit all the time, but most internet creators I follow are still on Twitter, so I can't switch fully to Mastodon just yet.