I think what is important here in the OP is the "at least give me a few options". Amongst my group of friends there has been a "quit facebook" movement in the last 6 months, and I can see why given the privacy issues that have been going on there. I probably would have joined them if I didn't live so far away from family and friends now. But really, it'd be nice to have a few options; lots of social sites allow you to use them to authenticate elsewhere, and things like OpenID which you can get from other places as well as rolling your own if you want to.
The only time you should be really concerned about whether your user really is who they say they are is if you are (or you are interacting with) some sort of official government or similar entity that legally and officially requires proof of identity. Otherwise it should be perfectly acceptable to have multiple, disparate identities on the internet.
The only time you should be really concerned about whether your user really is who they say they are is if you are (or you are interacting with) some sort of official government or similar entity that legally and officially requires proof of identity. Otherwise it should be perfectly acceptable to have multiple, disparate identities on the internet.