"Open" adoption can mean many different things, as folks above have pointed out. We have friends who adopted a baby with a truly OPEN adoption as you describe: friends on facebook, weekly phone calls, one major holiday per year with the extended birth family (and, yes, they bring all of their children), etc. I think it works because the birth mom was a young teenager when she had the kid (15), so there was just no question about how things should proceed from the perspective of the girl or her family... They truly have no regrets. Our friends just went to the birth mother's college graduation a few months back and there were apparently a lot of tears all around about how none of it would have been possible but for my friends and her knowledge that her birth child (now 7) was thriving.
Also, the birth mother and her family aren't "messed up" in any way, for lack of a better term. (Truthfully, in 90% of cases, this would have been an abortion, but the family was just religious enough for that to make them queasy, but just liberal enough not to be embarrassed to be public about the pregnancy/adoption.)