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 Zillow, (ZG 0.02%) fell 10% Friday after fellow online real estate specialist Trulia (NYSE: TRLA) turned in disappointing quarterly results.

So what: Keep in mind that Zillow already reported its own solid fourth-quarter results Wednesday after the market close. However, it remained guilty by association with Trulia, which plunged nearly 18% today after missing earnings expectations, and detailing plans for a new $45 million marketing campaign encompassing TV, radio, online, and mobile ads. 

Now what: While some analysts question whether Trulia is spending too much on its campaign relative to the size of its business -- which is almost three times smaller than Zillow in terms of market capitalization -- the new ads have also sparked some concern, because they'll likely be pitted directly against those from Zillow.

But I still don't think Zillow investors should read too far into today's drop, especially considering this kind of competition is par for the course in the real estate industry. In my view, Zillow is still the market leader in the online space, and it'll take a lot more than a competing advertising campaign to oust it from its position.

If that wasn't enough, remember that on Wednesday evening, Zillow management also said they'd be increasing their own advertising investments to $65 million across all channels this year, including a fresh push in TV after intentionally advertising lightly in the medium during its seasonally slow Q4.

I think today's pullback for Zillow is little more than a knee-jerk overreaction, and could be a fantastic buying opportunity for patient long-term investors.