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) went public today and promptly popped by as much as 18%, as investors couldn't wait to get their hands on the social networker.

So what: The IPO priced at $38 last night, on the high end of the expected range. Investors have been literally waiting for years for Facebook's public debut, and demand was robust heading into today as its first public trading day. Within the first minute of trading, shares briefly touched $45, only to quickly subside.

Now what: That glow didn't last long, as shares quickly trended lower to touch the offer price of $38, and then moved higher to trade within a relatively narrow range between $40 and $41 for most of the day. It didn't help that the Nasdaq (Nasdaq: NDAQ) was seeing some glitches and delays handling all of the volume, which sent its own shares down. Investors are now trying to figure out whether Facebook can justify its lofty valuation of more than $100 billion. By most accounts, Facebook's first day trading has been relatively modest, especially relative to the hype it was seeing leading up to the IPO.

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Fool contributor Evan Niu holds no position in any company mentioned. Check out his holdings and a short bio. The Motley Fool has a disclosure policy. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.