I have something simple right now, userdb schema is:

userid - autoincrement id email
email address
password

I want to incorporate Facebook and twitter, how would i deal with it on the DB side?
You can do this many ways, either you store most of the data in a generic usertable (as you are about to) and the provider details separated.
Or you make a design where you can connect multiple logins to same user. This will end up with something like

id user
id facebookuser (nullable)
id twitteruser (nullable)

This will maybe get you N many e-mail adresses (and still no password! since you arent the provider of the account); or none at all. It depends how much this user trust you in each provider.
Edit:
You might also want to normalize the data without nullables.
You can do this by having

id_user
id_facebookuser id_user
id_twitteruser id_user
Incorporating multiple login systems?