First, you need to research on all existing analytical data for Facebook so as to establish a price for each option contract or share of stock. Once done, contact a person that presently holds stock options and this will either be a present employee of Facebook or a person allied with a private equity firm who owns Facebook stock or stock options. Find out more details on http://www.ehow.com/how_5475015_buy-facebook-stock.html.
Go to an online stock market and buy! However, I am not sure if Mark Z (founder of FB) has released the sale of FB stocks yet. There will be 337 million shares and will be going up for sale with the set sale price. Which will be somewhere between 28 and 35 dollars per share.
Setting aside the fact that the share price has continuously declined since the IPO - wait for 6 months before deciding to buy. In 6 months Facebook will have had to make at least one quarterly report to shareholders and the disclosure that follows...a dynamic that didn't exist until about 6 days ago. And you will be able to gauge how the insiders really feel about the company by seeing whether they are buying or selling.