Facebook is going public. That means that you can own part of Facebook. Pretty cool huh? Well, it's not all good. In any start up company venture capital and angel investors (both are groups of rich people) only invest when they feel that the company will go up greatly in value and have an exit strategy. Going public through an IPO means the largest investors in the company are typically selling some of their shares in the company to get more money. In other words, the most informed investors about a particular company are selling their stock.
That is not strictly true for Facebook. When a company has over 500 shareholders they have to release their financials because the SEC mandates it. Facebook passed that threshold and since they will have to publish their financials anyway, they might as well make $18 billion off of it through an IPO.
That being said, I'm still thinking about buying some. I do hope that Twitter doesn't follow in Facebook's steps. Twitter is cool because it is simple. It is actually my prefered social media at the moment.