What is the purpose of buying facebook stock?

[Kim]

Buying stock gets you three things: you get dividends (a part of the company's profits), you get to vote on company policy at shareholder's meetings, and you can sell the stock for a profit if the price goes up.

The reason you'd want Facebook stock over others is because you either want to have a say in Facebook policy specifically, or because you imagine it'll be a betters investment of your money. People who thought the latter thing were wrong.