I see stuff on SoundCloud like people recording guitar to a karaoke track and haven't seen it banned or pulled from the site.
My paranoia on this topic is as follows:
1) there have been news blurbs about more and more crackdowns on small-time offenders (restaurants that host live music, but didn't pay ASCAP or BMI). Success makes sharks bolder
2) it has always been the conventional wisdom that a little guy without deep pockets is not worth the trouble of a lawsuit. But the web may have changed that.
All they'd have to do is subpoena all the music sites to obtain those sign-up agreements, then initiate a class action lawsuit. Getting a little bit of money from a lot of people is almost as good as getting a lot of money from one offender.
I expect to see this eventually, because:
1) the records are in place
2) the technology exists to get them easily
3) and there's money to be made if they do