This recent story relates: http://news.ycombinator.com/comments?id=824
When reddit added comments, I think that was one of the complaints. But Digg's awful "@userX..." kludge demonstrates that people will flame no matter what, but if comments are unthreaded, they'll just flame more clumsily and less avoidably.
Whatever the site may offer, it's *usually* a plus for a user to be able to interact with other users. It becomes a burden when it's centered around a social community that no one seems to use. It's difficult to manage your contacts from so many of your different online identities.
So judge it based on your app. Is the hassle of adding your friends to the site going to be far smaller than the benefit?
Good point. If it's more useful to you to have more of your friends on the site then you will try to recruit them. But if it's still not vauable at all to them, then they will just think you are sending them junkmail. I think the most thriving online communities are the ones that grow organically. History shows us, it doesn't matter what the tool is, if people can use it to communicate with one another, they will find a way.