I'm not sure those days ever existed. MySpace is still operating, after all. I think we may have overestimated the degree to which network effects lead to an exclusive or monopolistic structure - it turns out a lot of people are pretty willing to have multiple active social media accounts, and that there are disadvantages to very large networks (pushing people to spread out into a series of smaller ones). And then there's federated stuff like Discord and Mastodon, where you can use the same account to join many networks, decreasing the modest friction.