It doesn't seem likely that Instagram or Twitter is going to go away tomorrow. Posting to Twitter and Mastodon means that eventually there would be content and interactions building up over time.

There seem to be two problems with replacing Instagram or Twitter. The first one is money to run servers, fight spam, handle subpoenas, etc.

The other one is having an authoritative directory. How is anyone to know that "ClearAndPresent" on Twitter is the same as "ClearAndPresent@SomeRandomInstance.mastodon"? An obvious way would be to send a DM to a Twitter account but it seems likely that Twitter would shut that down pretty quickly. We've all heard the stories of Facebook (and LinkedIn and ...) pillaging Gmail for contacts.

I actually have my twitter and mastodon accounts linked with this, but I assume a bridge like the other commenter requested would allow to follow any twitter acount from your Mastodon account.