Zuckerberg's original intention was to create a dating website for colleges. It was obvious to eyeball this intent from the pre-wall days. Your page looked like this:

I am a: [man]

Interested in: [women]

Relationship status: [single]

Fave quotes, fave music, about me.

Then, all that got put on hold as FB was built out in every way except that (including adding the Wall, and then the News Feed). They must have realized that FB could be something much larger than a dating network, that would get used indefinitely by its customers, without pigeonholing their primary use case.

I'm surprised it took this long to pull the trigger on this feature, though.