It's been there since the 90's and early 00's. It's called HTML and RSS. If you put it on a server that's slightly more under your control than 'the cloud', than you're even more golden. Just try it, it's fun.
I feel like RSS / HTML fail to make easy certain components of the social media experience. They don't do a particularly good job of assisting networking, which I would consider a primary function of social media, nor do they do very well with content discovery. Any implementation that's going to displace the fbs and twits of the world is going to have to have some mechanism to solve that problem.
Perhaps something to augment RSS for discoverability and internetworking is the ticket. It's not a fully-baked thought, though.
A protocol is an API, not frontend (HTML/CSS). You having your own social network amounts to one protocol, but it can’t compete with Facebook unless other people’s custom social network can exchange friends/followers with yours. For which a standardized protocol is needed.
