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 car-sharing-service pioneer Zipcar (Nasdaq: ZIP) were zipping in reverse today, falling as much as 13% in intraday trading after the company reported first-quarter results.

So what: For the quarter, Zipcar managed 20% year-over-year growth in revenue, to $59 million, which was roughly in line with what Wall Street analysts were expecting. On the bottom line, the company significantly improved on the $0.95 per-share loss of last year, losing just $0.08 this time around. Analysts had estimated that the company would lose $0.10 per share. The growth was driven by a 23% jump in membership and the company boasted more than 709,000 members at the end of the quarter.

That all sounds pretty good so far, right?

Now what: For a stock with an exciting story like Zipcar, early investors are going to be looking for growth -- and lots of it. To be sure, the company continues to grow nicely and it's projecting a full-year profit, but it looks like that profit will miss what Wall Street was hoping for. With projected full-year profit of $3 million-$7 million, the midpoint earnings-per-share forecast would fall at around $0.125. Current estimates were looking for $0.14.

This isn't good news, but it's also hardly the end of the world for Zipcar or its investors. If Zipcar is able to deliver on the long-term promise that it's gunning for, the payoff for investors isn't in a couple pennies in earnings per share this year. Rather, it's about the execution by the company and the development of this segment of the car-rental market in the years ahead.

Want to keep up to date on Zipcar? Add it to your watchlist.