Twitter is a very simple app.

There are companies that run things of Twitter's complexity level but with ten or less engineers. It is perfectly feasible. You may see such things in payment processing, for example.

Then upon that Twitter does lots of analytics for advertising, manipulation/censorship, part of which is a regulatory requirement but most of which is intentional and for the benefit of Twitter's owners and advertisers. That was their major business, and that's why their activity has ballooned.

It's entirely possible to write a small distributed Twitter-like computer program that acts like a mesh network and is user controlled. Many such programs exist and function perfectly, but have not reached critical mass.