Facebook has launched a new registration tool that enables websites to offer quick and easy social options for users to sign-up.
Check out this reasonably detailed overview for more. This is of interest to startups because it may allow you to dispense with a lot of typical signup form fields, making the sign-up process easier (once it's popular, anyway).
It may be worth waiting a few months at least before implementing this, however, in case it doesn't catch on.