This week I finally got around to watching the Frontline documentary "Growing Up Online," which is actually available for viewing online, and I'm seeing the dark side of online communities as third spaces. Although I'm able to cut through some of the paranoia about teenagers discovering themselves online, a legitimate point is that members of a community can turn against each other, and for better or worse, people say things online that they wouldn't say in person.
I don't think this completely negates the idea of online community as a third place, but it adds another dimension. Also, the documentary focuses on adolescents, a notoriously fickle group that does not necessarily commit to a third place in the way an adult does.
But a third place is somewhere appealing where you are welcomed rather than harassed, and an online community can only be a third space where this holds true. And I do think that most online communities are positive environments, or people wouldn't be attracted to go there. But it's interesting to think about how the equation could go wrong.