When Facebook (FB) earnings were released after the bell yesterday, FB stock investors sent the shares up a sizzling 15%.

But few troubling statements on the Facebook earnings conference call cooled off the rally. And in today’s trading, FB stock is only up a little over 3%.

Facebook earnings were solid, as FB posted a profit of 25 cents a share — above the Street forecast of 19 cents. And revenue soared by 60% to $2.02 billion, which was above the consensus estimate of $1.91 billion.

FB founder and CEO Mark Zuckerberg has been aggressive with pushing mobile ads, which certainly paid off in Q3. Consider that about 49% of overall ad FB revenues came from mobile sources, up from 41% in the second quarter. And a year ago it was essentially zero.

But to at least some extent, much of that growth has already been factored in to FB. Heck, Facebook stock is up 90% year-to-date and more than 140% over the last 12 months.

And beyond that, management admitted on the Facebook earnings call that the future could be a bit murky. One big issue is that ads can be much more annoying on mobile than they are on desktop. On a larger screen, Facebook can place ads off to the side. But with mobile, the small form factor means that ads can easily crowd out other content.

But FB CFO David Ebersman noted that there will probably not be an increase in the frequency of ads, and that analysts should factor this into their earnings projections. That means Facebook will need to find new users and increase engagement from existing ones to keep up the growth momentum.

That’s challenging, especially considering it seems like teens are getting bored with the FB service. With Facebook becoming mainstream — and popular with parents and grandparents — it has lost its “cool” factor. As a result, teens are flocking to other apps like Snapchat and WhatsApp.

So while Zuckerberg has done a great job transforming his company into a mobile juggernaut, his challenges are far from over. The irony is that the ascent of Facebook was the result of the downfall of MySpace, which also lost its coolness. And the tech space is full of such examples, as seen with Zynga (ZNGA), AOL (AOL) and Yahoo (YHOO). The deterioration can be rapid and brutal.

With that in mind, it’s definitely understandable that Wall Street is jittery about Facebook stock.

Tom Taulli runs the InvestorPlace blog IPO Playbook. He is also the author of High-Profit IPO Strategies, All About Commodities and All About Short Selling. Follow him on Twitter at @ttaulli. As of this writing, he did not hold a position in any of the aforementioned securities.