I'm not so sure about that line of reasoning; you're focusing on the size of the community (and sure, small communities online can be extremely dangerous!) but

I'm not so sure about that line of reasoning; you're focusing on the size of the community (and sure, small communities online can be extremely dangerous!) but the nasty examples you're likely thinking of allow self-selection within a larger group. However, the kind of decentralization that follows local groups and families is different; you don't get to ignore that bigoted great-uncle (nor he get to ignore his crazy communist niece). If decentralization followed national or regional control as you'd need to satisfy conflicting courts, you're more likely to trend towards the latter kind of smaller community, not the former. The size of the community is probably less relevant that how it is grown. Self-selection (like e.g. hackernews) is certainly much more vulnerable to groupthink.