First try answering who you are? or who I'm? Let me take the task of identifying the person who is named "vijesh". I'm being dumb and dependent on (my favorite) search engine Google. I type in "vijesh" (quotes for clarity) in the search box and hit search. The screen reloads in 0.04 sec. Look my blog is there at the second place as on this date. Scanning down, there is one that points to my blogger profile page, that gives some more information about me.
Here is an interesting example of my friend or rather many of them did. When her dad sends them a matrimony profile, via email. Simple! they just log into Orkut (atleast its popular in India). Use the search effectively, spell the groom name and his city correctly to find the target. Ok fine, after getting the profile, go to the communities section and see which company interest group he is a part of. This will cross verify if he's the right groom. Next scan his photos, then his scrapbook, scrutinies his friends and their scrap for his reply. :) His testimonials. (you can't guess how deep the rabbit hole is...) Next get hold of her friend in the same organization and ask them if they knew some details about him.
This way, and more interesting ways an online identity of a person can be sketched. What is his/her interest? What is their favorite, their location based information, their employer (Linked In). Using such public data, one can form a digital image of any Y. Even worse they can know what one is doing right now if they are active in twitter. Online spiders could use this identity for making the online experience better.
Humm, All of them are voluntary by the person. It is oneself who is making these data public and explicit. So we don't have anyone to blame about. So should there be a trade off between whats public and whats private? Maybe yes! Understand a simple phenomenon, when its there for any bot to find, then anyone (yes literal sense anyone who can connect to the Internet) can view your details. So you can consistently use a nick name! Can you recollect how many user accounts you have created in those numerous number of social networking sites, web portals, user accounts to download software? I cannot even guess a number. Its so much, even maintaining passwords has become a pain in the ass. Thanks to those openID's its much more simple now.
Try this, take a paper and do all hacks known to get your identity yourself. You will be surprised to note the amount of details.
So be responsible, know what you are on the Internet.