I have in both but I am using Facebook more than twitter as I don't understand how the twitters work. I guess, twitter is good if you want to promoting your site, though some told me that they met new friends there as well (I am not sure) and Facebook works in two ways you can promote your site and socialize and met new friends.
if i were to choose between the two i will choose facebook because the system on twitter is not that good it only shows just a fan-idol cycle, if you were famous you have so many follower but if you are not, what oyu can do is just follow.Unlike on facebook they have fair system no fan no idol,just a friend status