Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Facebook (NASDAQ:FB) have popped today by as much as 13% after the lockup on shares related to the company's IPO expired.

So what: Stocks typically see some selling pressure when lockups expire, as early investors are free to unload at will and supply increases. Facebook sold off by over 6% in August on the first of its lockup expirations, but today is playing out quite differently. Approximately 804 million shares are now eligible for sale, but it seems those shareholders aren't selling.

Now what: The lack of selling pressure is giving bulls some fuel today. Since shares were previously trading closer to the lows, early investors may view prices as too low and be unwilling to sell. There's also likely some short covering taking place, as bearish investors buy shares to close out their positions. This batch was the largest of several expirations, but there's still another expiration coming in December and another next May. Want a full breakdown on what's going on? Click here to read more.

Fool contributor Evan Niu, CFA, has no positions in the stocks mentioned above. The Motley Fool owns shares of Facebook and has the following options: long JAN 2014 $20.00 calls on Facebook. Motley Fool newsletter services recommend Facebook. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.