I have seen a couple of instances of strong web communities (one on ning, one on heaven-knows-what forum software) stagnate and die because everyone is reinventing the wheel with groups on Facebook.
The Facebook version is much less serviceable, primarily because the content is so ephemeral, and not searchable, but there are also issues with ownership of content.
The community that I am trying to foster (on Drupal + CiviCRM) is at an even greater disadvantage because in eight years it has never reached a high level of engagement in a non-technical sector.