One that I only log into about once a month when I want to look at someone else's page from a search result. I feel a bit guilty about not keeping in contact with my friends more often, but Facebook just looks like such a dreadful time sink.
I've got roughly the same use patterns as you, and if it weren't for the one or two regular events that I attend that are organized on Facebook, I'd probably delete my account. And yes, it is a dreadful time sink. That's why after the first flush I scaled back to around once a month.