There is a way to buy (and trade) pre-IPO stock in private companies. One of sites is called Sharepost.
https://welcome.sharespost.com/ . FB was auctioning shares through them before the IPO and the shares were going to $28 - $38 per share and the minimum investment was $100K. I found these guys after the LinkedIn IPO. You have to be an acredited investor to invest with them (minimum net worth requirement and a salary threshhold). I will post their intro statement in the next post.