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There are a bunch of "hard parts" involved, and even if you solve them, you don't replicate the experience of a social media site.

The serving technology is hard.

The dollar cost is high.

The maintenance burden is high.

The authoring experience sucks.

The notification / distribution mechanisms are lacking.

The feedback loops are broken.

Social media sites are simple, free, zero-maintenance, trivial to post, notify/distribute to your friends and others, and build in a feedback system with both automated and human curation.

There's an enormous amount of effort required to provide even a remotely similar level of service.