The down side is that you will be logging in with the Facebook id (your real name). For some, that may be a privacy concern. In addition, the activity you partake in at those other sites will appear on your Facebook account. Again, some may see that as a privacy issue.

Finally, my biggest concern is over the security of the username and password. Clearly, if you don't take great care of this, and perhaps utilise a high strength password, or it gets hacked, your identity may be compromised in more than one location.

I guess this also applies to any open ID, such as Google.