You forget about the network effect. None of our friends are on IRC. Reddit is great for some niche groups, but there are tons of facebook groups specific to a niche that really have no equivalent anywhere else.
The value being in the network means the value is subject to social whims. That's not a secure place to do business long term. All other places I listed have networks as well. You have chosen to think this network is better for you, but if/when people move then you might need to re-evaluate that as well.
