Facebook founder and CEO Mark Zuckerberg announced earlier this week that he was planning to put $100 million of his personal stock into a foundation to be used toward saving the Newark Public Schools. The pledge sparked arguments over the timing and motivation, but it also reignited an old debate: How much is Facebook actually worth?

Zuckerberg, whose net worth was recently estimated at $6.9 billion by Forbes, owns about a quarter of the social networking site. The rest of the company is also in private hands, so there's no clear way to determine value.

But that hasn't stopped people from trying.

Putting a price on Facebook -- and other privately held dot-coms -- is a game with a long and fairly complicated history. A handful of groups -- SecondMarket and SecondShares are two of the most prominent -- have sprung up that are solely dedicated to trading restricted and illiquid assets of private companies or watching and analyzing those trades.

There's obviously a lot of room for debate, but we've scoured the Internet and talked to experts to put together a list of valuations for some of your favorite social media sites.