Aside from reconciling with my long lost friends on FB, i am playing Zynga Texas Holdem Poker. It is so addicting specially when winning. I also play cityville as my second choice. But Poker rules!
Mafia Wars hands down... its very addicting .i play this game too and you so right i sometimes spend hours without knowing .i get surprised when it gets dark without knowing.
I haven't played a lot of games on facebook but I liked crazy taxi and it becomes interesting when you have to compete with your friends (their scores).