Why do we not fear a web presence?

I am astounded at the incredible success of sites like Blogger.com, MySpace and Facebook. Utterly astounded.
Why are we so willing to post pictures of ourselves (often at our worst) to the internet where anyone in the world can access them?
Why is someone willing to wax philosphical to complete strangers and even open his or her life to the scrutiny of others in this way? We don't treat our homes this way, do we? We are often very guarded about who we let into our personal life, who we allow in our home, what information we are willing to let out about ourselves. But the internet is a free-for-all, especially sites like Facebook or MySpace. I can be careful about what I post on my blog, carefully consider my words, make myself look intelligent, charismatic, whatever I choose, but if someone wants to "tag" me in a photo on Facebook, I am there, for better or worse, for the entire world to see.
This hasn't bothered me until just now.
Why is this?
I know that nobody subscribes to my blog and I am pretty sure that only an occasional lost blogger ever looks at my blog, so I feel some sense of security while remaining open and vulnerable on line. So why do I even post?
I had some semi-active readers when I began this endeavor, but that was because I read their blogs. Once I stopped posting and visiting their blogs, they stopped visiting mine.
Now nobody bothers.
Maybe that's for the best.