Most sites, at least dice sites, provide either a login url (Just-Dice, SatoshiDice) or ask for a user name and password (BitDice, PrimeDice).
One of the benefits of Bitcoin gambling is betting anonymously.
There has been a couple of sites however that allowed you to login with your social media profiles, though they seem to be dead now
Do you think sites should offer the option of login via social media profile as well as the normal username/password?
Many gambling sites are operating without governments permission, and are either illegal or in the gray area, so players usually try to preserve their anonymity. So, logging in via social networks is the direct opposite of this and should be avoided. Also, your profile information from social networks can be used by hackers in their social engineering attacks, you can find many stories how people got their exchange accounts hacked because of this. Cryptocurrencies gave us tools to reach anonymity, and we shouldn't neglect it.