Facebook succeeds because the open web is a UX nightmare for anyone without a tech background. FB is the usccessor to AOL is the successor to [prodify, genie, whichever] is the successor to text-based bulletin board software.
People primarily use social media to connect with and make new friends. They do not want to deal with a million and 1 different user interfaces in the name of openness. That's why Usenet worked so well until people like Canter & siegel killed it with commercial spam.
The open web is great for the same reason that magazines and TV are great, but it's also shit for the same reason that magazines and TV are shit - too much shallow design competing for people's attention, with everyone trying to differentiate themselves from everyone else via style and gimmicks. That's the same thing that made myspace unusual, an impossible excess of visual cruft that far outweighed its substantive communication value.
The open web is fine, but the web is not the right platform for social networking. For social you want a fast, clean, boring interface...like the one on HN. If you think FB is the problem (and it's very very problematic but plenty of other people are already making that case), stop rabbiting about web standards and get to work on NNTP 2.0.
NNTP was fucking awesome. Gopher was fucking awesome. the semantic web is/was awesome. The web is not for social.