Facebook isn't invite-only, it's a bucket of shit but still works alright
99% of functioning communities online aren't invite-only, and work fine.
The reason for that is active contributions by users as well as the mod team to regulate the type of content available. You can increase the restrictions like adding captchas, human-checked questions, moderating applications, but invite-only systems are bad and anyone suggesting them should feel bad.
Dribble isn't nearly as popular a site as you'd think, it's extremely designer-centric but it's about the only example of a workable invite-only site. Secret question/answers however are a good filter I think - but they aren't really an invite system
The reason for that is active contributions by users as well as the mod team to regulate the type of content available. You can increase the restrictions like adding captchas, human-checked questions, moderating applications, but invite-only systems are bad and anyone suggesting them should feel bad.