that's an interesting idea.
connecting to those services I believe I can do but would take a bit of work maybe -- just never done anything like it. With G for example OAuth2 means you have to have an HTML frame or popup, (sorry, haven't read the other suggestions yet including this HTML 'box' thing,) but to do so with an internal browser and get the whole token access thing to work would be sketchy at best even if I could get it to work (the internal browser support is extremely minimal, supporting only basic browser technologies is not only easy for me but frequently an advantage for you on a system where too much junk is screwing up the default browser...
Anyway, a default browser popup to login is fine and dandy, but the access token does expire... I'm not sure on frequency and I guess different services, different whatever, or maybe it is standard I have no clue yet, but either way how many times do you want your user to login to Facebook when they launch your app? I guess if that is ok with you it is great by me.
The other thing is I'm just not wrapping my head around the management aspect of using social media accounts vs. a local or self-managed database style system, and how integration would actually work plus be beneficial.
btw as for the current Client ID system, I am thinking of designs to enhance (or more like replace) as I type this. More on that in a bit. Just dropping in to let you know I'm paying attention here.