I've noticed the best communities are often built by following the path of least resistance. Let's say you run a group which meets in person once a week. You'd like to create a way to inform everyone where the meeting is and allow for basic chatter. In this case, the simplest solution is a Facebook group. Almost everyone has FB and uses it at least once a week. This ensures your community gets all necessary information while providing the lowest barrier to entry.
If you had a group that met daily and engaged in heavy conversation outside of meetings, a facebook page would quickly become confusing (due to non-nested replies) and difficult to navigate (due to lackluster search and sorting of previous posts) and in this case a stand alone forum would be ideal.
If you mix and match these two scenarios, a forum for a low discussion weekly meeting might seem like over kill and eventually you would lose visitors because they just don't want to add another site to their current rotation.
The most efficient way to host an online community is to find your current and most basic community requirements and pick a host based on them. Don't focus on what your community could be until you've accounted for what your community is.