Using Facebook (or GMail) credentials to authenticate to other sites also carries another risk. If someone gets your username and password, they can also access all those sites where you signed up with the same credentials. For this reason, I would strongly recommend anyone doing this to use a two-factor authentication such as theone that Google offers.
I guess it also depends on the balance between easy signin and privacy. I started using Facebook credentials for a bike application that tracks my trips, and it works well. But I would not let just any application use my Facebook credentials to authenticate me.