As a recipient of publicity, rather than a publicizer, myself, here's my 2 cents:

I've found MySpace to be useful for listening to samples of music and a summary of what people are about. I don't hang out there, so I go there if I've googled on a performer looking for more info, or sometimes if I encounter a link elsewhere.

I do hang out on Facebook. For groups or individual performers that I'm interested in, it's useful to get reminders of their events via Facebook and/or group e-mails via Facebook. I've also used Facebook pages to learn more about performers and to find links to websites and MySpace pages. I've investigated groups and individuals that my Facebook friends have linked up with; I think that network can be very effective.

On Facebook, I don't want daily reports from those groups/individuals, however, and if I feel they're clogging up my channel, I'll hide them, which defeats their purpose. (I don't know if that affects e-mail missives or not...) So, I'd say that a sparing presence is probably a good idea.

~ Becky in Long Beach
a person of middle age, YMMV depending on the audience you seek...