On twitter/fb, people can look up your account by phone number. This is really fucking stupid because I only added my phone number for two factor authentication. So now I have to choose between security or people finding my dumb throwaway accounts.
You can disable this functionality in the privacy settings for Twitter-- not sure if Facebook allows it, though. Either way I totally agree that it's a dumb feature to have turned on by default.
It can be disabled (and at least in Twitter's case, I remembered it asked me if I want to enable it).