Max, most of the features you list have real downsides, and would probably come at the cost of advertising, slow-loading graphics, big-brotherish data collection and other spamming/intrusive opportunities that are the primary reasons I avoid the sites you list. Once you sell your soul to the devil, there's little chance of getting it back.

I view Mudcat less as a "social network" than as an information source. IMHO, PMs and chat are sufficient "social" features.
Very few people use the multi-chat feature, and when they do, they're seldom discussing music. This indicates the level of interest in "social" features. When people desire more direct interaction, they can easily move to regular email or an IM/videochat service; there is no compelling reason to include them here.

It would be nice to include sound clips, sheet music scans or MIDIs in posts, without moderator intervention, but there are understandable issues with that. Also an archive location for helpful utilities (like my HTML character conversion scripts).

IMs would encourage people to go "private" instead of sharing information (though many comments would be better sent as PMs). BBSs and public chat sites have degenerated over the years, primarily because of IM'ing.

The notification feature is already provided in a sense through trace; rather than emailing notifications, I would prefer that the trace page show a count of the messages added to each thread since I last read them.

I appreciate that it's easier to embed a canned service than to program in each feature, but this is one site where "less is more" (and more is less).