There are several ideas on how to simulate social network experience with classical w3c tech.
Linkeddata group (one with Tim Berners-Lee) work on SoLiD - "for developers who plan to build social linked data servers and applications". As you said, there is plenty of ways to make online social experience with original WWW methods, and it'll be distributed, and ad free, you just need to make a "pretty wrapper" for common folks.
If more developers acknowledged possibilities of semantic web (or "Linked Data" approach as it been re-branded) not just for reducing entropy of the web, but also to provide solid services for ordinary users, it wouldn't be so hard to develop critical mass to be widely used.
If more developers acknowledged possibilities of semantic web (or "Linked Data" approach as it been re-branded) not just for reducing entropy of the web, but also to provide solid services for ordinary users, it wouldn't be so hard to develop critical mass to be widely used.
[0] https://github.com/linkeddata/SoLiD