Susan Moore and James Baker, from Swinburne University of Technology, Melbourne, ran two consecutive studies. Their findings, as reported in this ABC Science Online article:
"We found potential bloggers were less satisfied with their friendships and they felt less socially integrated, they didn't feel as much part of a community as the people who weren't interested in blogging," Ms Moore said.
..."It was as if they were saying 'I'm going to do this blogging and it's going to help me'."[In a follow up study] Bloggers reported a greater sense of belonging to a group of like-minded people and feeling more confident they could rely on others for help.
The effect happens even if you're not blogging, and just hanging on out on BeMyFacebook:
All respondents, whether or not they blogged, reported feeling less anxious, depressed and stressed after two months of online social networking.
Of course, this is all based on the kind of teeny-tiny sample size (134 self-selected respondents on the first survey; 59 on the second).
p.s. thank you for being my friend.