can't login my jomsocial site with facebook credentials

That's not possible. There's no way to get a user's Facebook password, so there's no way to let them use the Joomla login form to validate their Facebook credentials.

If you want them to be able to login using Facebook, they have to use the Facebook login button. That's a requirement of Facebook itself and not something we can work around. The same restrictions exist for Google, Twitter, LinkedIn and every other social network due to security reasons.

I hope that helps explain,
Alex