Take this for what it is (a single anecdotal datapoint), but while I have both a Twitter account and a Facebook account (and an OpenID), I still prefer logging in with site-specific usernames. If your service is really compelling, I'd possibly use my Twitter or Facebook accounts to log in, but I've passed on other services in the past (at the point of signup) that used Twitter and Facebook auth (and, ostensibly, they used those auth methods with the express intent of eliminating friction).
