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 Chinese Internet portal SINA (Nasdaq: SINA) are sinking today, falling as much as 10% on heavy volume.

So what: There's a lot going on here: (1) SINA's fourth-quarter results were satisfying, but the outlook for the next quarter disappointed analysts; (2) one analyst firm upgraded the stock from "hold" to "buy," and another went in the opposite direction; (3) SINA is spending $66 million on a 19% stake in Chinese fashion retail portal Mecox Lane (Nasdaq: MCOX).

Now what: That's a lot of news to digest, but investors clearly focused more on the bad than the good. But is that fair? Conservative guidance doesn't mean much for a company in the habit of beating expectations, as SINA is, and Mecox Lane is a profitable cash machine with serious growth that should add to SINA's earnings right away. This looks like a knee-jerk overreaction and a potential buy-in point for this longtime Stock Advisor pick.

Interested in more info on SINA? Add it to your watchlist.

Fool contributor Anders Bylund holds no position in any of the companies discussed here. SINA is a Motley Fool Stock Advisor recommendation. 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 is investors writing for investors.