You don't need an alias to be hidden on the Internet. Even if you use your own first (or even full) name, who's going to know it's you? If you're from the US and I'm from New Zealand, how is knowing your real name going to make you more familiar to me? You're still a faceless, voiceless, odourless collection of text on a screen, hailing from a physical existance far away in a place half way across the world. On the other hand, as you rightly point out, if you choose to hide your real name it can still be found out by various means.

No, I don't think aliases are about hiding at all. I think, if anything, that they are about creating a new identity and advertising that. About bringing new facets of yourself to the fore. If I meet someone on the net who uses the alias "Kitten", I can safely assume they are fond of cats. If the go by "Skywalker", they're likely a Star Wars fan. This helps round out a personality in this text based world in a way that "John Smith" never could.