I use existing accounts like gmail to use some applications. For example, my SE account.
That said, when it comes to social networks, I intentionally avoid logging into most things through Facebook. FB's a much more informal, and personal network, and between privacy issues and applications in the past which have been known to post as the user to their friend's facebook walls etc, I think a lot of others avoid dishing out their Facebook login. Gmail, or a generic email account on the other hand, that's a whole different story.
You still need to think about your audience and whether or not they'd even think it that far through. I'd suggest if you're going to allow users to login with other social networks, that you allow a wide variety so they aren't avoiding your app because they don't trust you with a Facebook login, while they would trust you with a gmail login.