All of the latter three sites did multiple things wrong:
1. Fail to compete with competitors
3. Fail to build a community
Issue 1 manifested itself differently for each site. Xanga couldn't keep up with LiveJournal, which not only arrived on the scene after Xanga but out-innovated them as well. MySpace was a clusterfuck of design and functionality that got stale while Facebook was improving constantly. Digg ignored the hell out of its users and simply had nothing compelling vs Reddit.
Also, none of the sites pulled in people and kept them. The narrative for MySpace users was that people were becoming "friends" with hundreds of people who they did not know. It doesn't take a genius to realize that this essentially invalidates the whole point of a social network. Xanga never got a large audience and also suffered from the same problem of anonymous users. Digg? Digg never even really tried. Shouts were pathetic.
Facebook solved these problems. While MySpace appeared to do nothing after News Corp acquired them, Facebok kept changing the site in a very public way--and even when they got negative publicity (e.g. privacy settings), they made sure that people knew they were actively developing the site. Additionally, users on Facebook know the people in their network, and users are constantly given a reason to come back (communicate with your friends! apps! interact with companies!). Faceboook survived because they made it clear that they are the best game in town.