Creating a custom PhoneGap plugin using Facebook/Twitter SDKs is what I normally do with my projects, though it could increase the size of overall app size.
If so, what version of phonegap are you already using? Kindly let me know.
On the login part - I want to make it clear that normally we save a newly created user_id at phone space to know what user is actually logged in.
After succesfful login process at Facebook/Twitter, before showing them the signup form, we can see that facebook/twitter user is already there on our server. if so, then just log them in instead of showing up the signup form. Let me know what are your thoughts on it please.
Thanks,
Sadi