In my new web app I'm going to allow social login through Twitter (maybe), Facebook and Google and I'm in search of the best way to do it.

Actually I'm using Rails with Devise + Omniauth and this is the problem:

Should I ask the user to choose a password so that he can login without a social network?

Or maybe the user should be able to set a password if he want (for example when editing his account?)

The second way seems the best one but since Twitter doesn't provide user email and google doesn't provide an username I'll probably have to ask the user for username/email when he log in so in that case I may also ask for the password...

waht do you think?