Yeah, I've come to the conclusion that social networking is over-rated for an artist - well for an ambient artist anyway. Particularly considering the time needed to maintain and update multiple profiles. I agree with Saul having your own site as a 'node' behind all these profiles is a good idea. I think the days of having (say) My space as a sole artist page without some kind of artist website may be coming to an end. Even with Soundcloud (which looks cool), I'm not convinced about the value of doing all the 'networking' that is needed to get things going.
Regarding exposure, I think airplay is more significant. Somafm, Stillstream and others all 'broadcasting' 24/7 every day certainly gets the music out there.
I also look at Lastfm too to see what is being listened to. I also created lots of videos for YouTube
as that seems to be where a lot of people are these days. Apart from updating the LuminaSounds site, I don't spend much time on the social networking sites for exposure these days.