The success or failure of any groups that form, via pretty much any tools, comes down to the same thing - how well are you enabling those involved to achieve the three things they join groups to do? (image courtesy Kbaird.)
Three Reasons People Join Groups:
1. Express themselves: Usually this is support for or antipathy toward something. It's how people know that group X is one that cares about the same stuff they do.
2. Discuss what needs fixing about it.
3. Fix itFacebook has the tools for peer-to-peer comms - but pages and groups veer towards broadcasting at an audience without extreme caution. Success depends on what the community is given and in how much they talk to each other. Communities talk to each other, audiences are broadcast at.
One way 'founders' of groups can encourage more interaction is by appointing interested folk to roles in which they can take more ownership of the group. Obviously ongoing support for the group through tools, content and interaction is critical , too.
Few groups get past point 1.
To point 3, make clear in the group how people joining can help and what they can actually do to manifest that.
Peer-to-peer interaction is always less about the viral marketing of a message, more about bringing people together who care, to talk - and then act.