More to the point though, twitter's issues tend not to be the kind of thing you're looking for in beta testing. You can't test scale with a limited number of users by defintion. A service which still loses data in corner cases, exposes private information unintentionally and so on is better off in an acknowledged private beta than suffering the incredibly bad press and resentment of users which will arise in these scenarios.
Also, not all software is built equal. Twitter is strongly affected by network effects, the more people using it, the more valuable it becomes, so getting people signed up is more important than ironing out every bug. However the likes of gmail is weakly affected as you can still email all of your mates whether they use gmail or not. Spotify's value doesn't particularly increase when new users join it, but it does when it's catalogue is expanded.