How much does online community count?

Good scientific evidence shows that people are happier and live longer when they have a strong social network, and even when they have a pet to take care of.
In this context, how much do online social networks count? When someone spends time blogging; participating in mailing list discussions; chatting on IRC; does this strengthen the immune system? Or weaken it? Does it make a difference if the participants meet each other in person every once in a while, or never?
Has anyone done research on this yet?