During this week’s earnings call for Yahoo (NASDAQ:YHOO), CEO Marissa Mayer made it clear that mobile will lead the company back to growth. In fact, Wall Street was actually encouraged. After years of meandering, Yahoo finally seems to have a vision.

And Mayer hasn’t wasted any time, even though she recently had a baby. Yesterday, she announced a deal to acquire Stamped, which is a mobile app developer. It’s a small deal, with a price tag rumored to be roughly $10 million.

Stamped launched its app back in November. Essentially, it allows users to make recommendations about restaurants, bars, movies, music and so on.

While the app didn’t get much traction, the company was able to attract a variety of interesting investors, including Justin Bieber, Ellen Degeneres and Ryan Seacrest.

So, why would Mayer want to buy Stamped? Simply put, it’s a quick way to pick up a group of mobile engineers. Consider that two of the co-founders were former Google (NASDAQ:GOOG) employees.

Ultimately, Mayer’s goal is to get about half of Yahoo’s technical people focused on mobile. While this will mean smart recruiting, a key will also be acquisitions. This is a common strategy in Silicon Valley, as seen with companies like Facebook (NASDAQ:FB), Twitter and Google.

But for Yahoo, there will be some huge challenges. For the most part, the company continues to operate at stall-speed. In the latest quarter, revenues inched up by only 2% to $1.08 billion. Keep in mind that the two core businesses, search and display ads, continue to lose market share. In fact, search revenue would have been lower if Microsoft (NASDAQ:MSFT) weren’t providing minimum payments.

The biggest problem is that Yahoo’s rivals are rapidly grabbing the mobile opportunity. For example, Facebook now has 600 million active users that come from smartphones and feature phones. It, too, is focusing hard on monetizing its mobile traffic, which is already showing traction.

Google is also a huge factor in mobile. Its Android platform is getting about 1.3 million activations per day and has a global installed base of over 500 million users. In terms of monetization, the run-rate is at a stunning $8 billion, which is up from $2.5 billion a year ago.

So, when advertisers look for options, they really have two compelling ones. Unfortunately for Yahoo, it doesn’t have the scale to get much interest right now.

In the meantime, the company will need to restructure its display-ad and search businesses as well as cut back on employees. It will be an incredibly tough juggling act.

Now, it’s certainly possible for Yahoo to stage a comeback. It has great assets, such as channels for weather, financial information and sports, which can be leveraged for mobile. But it will be extremely difficult to get an edge against players like Facebook and Google, which continue to relentlessly bolster their positions in the market.

All in all, playing catch-up is never a good strategy, especially when dealing with fierce rivals.

Tom Taulli runs the InvestorPlace blog IPOPlaybook, a site dedicated to the hottest news and rumors about initial public offerings. He is also the author of “How to Create the Next Facebook.”  Follow him on Twitter at @ttaulli. As of this writing, he did not hold a position in any of the aforementioned securities.