That's true: there is very little negativity on myspace, plenty of "thanks for the ad", and the odd "I like your space" or suchlike.
Friends are really, mostly, Links, of course, and perhaps users think that if they put something negative they will be deleted and lose out.
And such things are, of course, subjective: neither of the above lines from Mr. Smith or Amos do anything for me - save cause an itch.