But if you're using your social network to find the solution to your recurring plumbing problem that the last 3 plumbers you hired couldn't fix, then you'll probably be just as happy with an answer from a plumber from across the world as you would from an plumber next door.

The power of online social networks it that they aren't limited to whoever is in your town.

Otherwise it's still subject to all of the same DDoS attacks of a global social network provider. Except that it'll be easier to take it down since it has fewer resources to protect itself (i.e. it doesn't have multiple 10 gbit pipes to the internet and likely can't blackhole traffic with BGP routes)

So yes, the internal network may be fine if a DDoS attack takes down the router to the rest of the internet, but does that matter since no one can reach it?

Or are you envisioning entire communities unplugging themselves from the internet if any community member is under DoS attack?

A DDoS attack is dependent on flooding the users connection with traffic, so if this connection cannot be established, a DDoS is unable to have a significant impact