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 Trulia, (NYSE: TRLA) plunged nearly 18% Friday after the online real estate specialist turned in disappointing fourth-quarter results.

So what: Quarterly revenue rose 142% year over year to $49.7 million, which translated to adjusted net income of $0.03 per share. Analysts, on average, were expecting earnings of $0.08 per share on sales of $49.39 million.

Trulia also expects current-quarter revenue in the range of $53.1 million to $53.5 million, and full year 2014 revenue in the range of $245 million to $248 million. By comparison, analysts were looking for Q1 and full year revenue of $53.09 million and $245.23 million, respectively.

What's more, Trulia expects adjusted earnings before interest, taxes, depreciation and amortization to be in the range of $1.4 million to $1.6 million, or only 3% of revenue at the midpoint. For the full year, Trulia anticipates EBITDA to be in the range of $18 million to $22 million, or 8% of revenue at the midpoint.

By comparison, adjusted EBITDA in 2013 was $17.1 million, or 11.9% of revenue.

Now what: In the subsequent conference call, management suggested much of its lower profitability will stem from a $45 million marketing campaign encompassing TV, radio, online, and mobile ads. For a company Trulia's size, that not only seems like a huge amount of money, but also sparks concerns that such a campaign could potentially yield lower-quality leads than the ones to which the company is currently accustomed.

Even so, CEO Pete Flint insisted the company is confident in its ability to selectively use broad advertising to find high-quality customers and accelerate Trulia's growth.

As it stands, the stock doesn't look particularly cheap trading around 32 times this year's expected earnings, and eight times last year's sales. Keeping in mind both the fact that those estimates are likely to decrease, and the risks involved with such a large marketing campaign, I think investors would do well to wait on the sidelines with Trulia stock for now.