Richard Millington lists the advantages and disadvantages of using Facebook Groups. The fact that they are simple and tie into the existing habits of many people that visit Facebook every day are big advantages.
In communities of practice, unique interests, and many other fields, people want unique identities. Facebook doesn't allow that. Many members won't join your community because they don't want to combine their online identities. You might not want your Facebook friends to know you're a huge fan of Harry Potter fan fiction.