The thing I find interesting about blogging, is that it's sort of like sending a personal letter out to whomever wants to read it.  Personal blogs can seem very intimate even if they're selective in what they present.  As a consequence, I can read your blog, feel like I know you, yet if I choose not to comment or make my presence here known, you have no idea I exist. It's so odd.  You do a good job of engaging your readers in conversation here on your blog, so while I might lurk on other blogs, when I'm invited to weigh in on things, I usually do.  

Anyway, I think your point was about the social networking sites, but my point I guess is that, I might feel compelled to follow you on twitter because I've read your blog, and I think, "Oh that's Kristin, my divorced, liberal, Christian, mommy friend. I know her. I know the view from her front porch, and how she's going to talk to her kids about sex."  But unless I've commented on your blog or something, you probably have no idea who I am.