Here's link to an article with reasons companies give up doing their own internal "communities" and opt-for established sites like Facebook.
Reasons listed for building community on Facebook:
1. Built in audience;
2. Built in viral features;
3. Minimal community management hassle;
4. Minimal development costs;
5. Rapid roll-out.
With downside including:
1. Don't own it;
2. Limited to their platform features;
3. No data ownership (can send messages, but can't get email addresses);
4. Competitor can do same thing.