design - Asking user to create a direct account after registering from Facebook Connect / Twitter OAuth -

we're planning have sign-up page allows users use facebook connect or twitter sign in. after validate account , fetch information parent site, acceptable thing ask them create direct login/password our site?

i've not seen sites this, havent used fb-connect well...

acceptable? depends on users , expectations wonder why site using tactic wouldn't ask me create local account in first place.

generally, industry seems moving towards trusted third party user authentication, facebook, twitter or openid.

the value user of allowing sign-in application using third-party service (in has account) reduce number of accounts must keep track of, requiring create yet account seem make such benefit moot.

of course, there nothing stopping creating local user model hold custom user data authentication handled either via passwords or third party, in:

create table user (   user_id int primary key   name varchar,   validated_by enum('password','facebook','twitter', etc.)   /* other custom fields here */ );  create table password (   password_id int primary key,   user_id int foreign key,   value varchar ); 

and create business logic "do right thing" in cases.