To steal/paraphrase from Cory Doctorow: "Facebook started as a website to nonconsensually rate the fuckability of Harvard graduates, and it only got worse from there."
That's more of a meme though in fairness multiple things can be simultaneously true.
The original purpose was to gather data from people and map who is friends with who. A live diary so to speak. It's original name was LifeLog. Think tanks said it was too on the nose so they cancelled it and shortly thereafter Zuk was on the news announcing TheFaceBook. [1] Most of these platforms came out of and were indirectly funded by the dark side of Stanford SRI.