This is how TextSecure worked before it was renamed to Signal and removed SMS support.

There is a completely free, add-less social network: Mastodon. But for some obscure reason, “nobody” wants to use it. Instead, everyone is moving to BlueSky.

I don’t think costs are the reason for people not using certain social networks. Simplicity and ease-of-use is. And federated networks suffer hard from this, because simply explaining the way they work is too much for people. They want to read and post stuff, and not think about which server to pick so that they can get the biggest audience.