Social media apps should share the same social graph by implementing ActivityPub

The only other alternative suggestion I have is to make sure there is rdfa or microdata markup on all pages.

Rather than have an explicit protocol for social media apps, the idea is that anyone can do whatever they want: it's all just stuff on a page. Then we interoperate by writing scrapers that can "see" the standard posts, pictures, calendar events, marketplace postings, &c.

ActivityPub is probably a better basis to work from, all in all. There are some advantages though to making social media pages more semantically expressive, to decomposing the page from a big monolithic thing into a thing that has other things on it.

A quick reminder that the Fediverse is not a new concept at all - it recently celebrated its 13th birthday:

https://news.ycombinator.com/item?id=27201389