Facebook investors: don't make this mistake

Shares in Facebook look cheap from where I'm sitting. Since opening at $42 in the middle of May, they've plunged an astounding 45% and are trading at a little over $23 today. Certainly they must be a buy at this price, right?

Well, not so fast. If you've been tempted by this rationale, as we all have at one time or another in our investing careers, it's important to recognize that, as the old saying goes, looks can be deceiving. Sure, there's no denying that shares in the social-media giant may be cheap, but they aren't so simply because they're trading for almost half the debut price.