A Facebook employee holds a cell phone, on April 4, 2013, in Menlo Park, Calif. © Justin Sullivan, Getty Images

Facebook (FB) has doubled in just a few short months, sparked by better-than-expected second-quarter earnings in July, and is up over 170 percent in the last year.

And while it's unfashionable to badmouth this social-media stock as it approaches its next earnings report, count me among the Facebook bears.

I think that even if Facebook manages to post decent numbers this week when it reports earnings, the stock can't keep this run up for much longer — and investors who are sitting on big profits may be wise to sell now rather than risk steep declines.

Here's why I refuse to "like" Facebook stock.

Facebook's most valuable users flatlining

The biggest reason to be bearish on Facebook is that domestic and European users have flatlined. I personally expect the company to post a decline in its U.S./Canada segment this quarter, or next quarter at the very latest.

That is not going to be a pleasant headline when FB sees its first-ever drop in U.S. users after its massive growth… just look at the fireworks in Netflix (NFLX) after its subscriber drop at home as a case study.

This is disturbing for obvious reasons in regard to the saturation of these markets and how it will affect growth, but it's even more disturbing when you consider that growth in emerging markets comes with only a fraction of the revenue.

Consider these figures from Facebook's last earnings report:

U.S./Canada

  • Users: 198 million, or 17.1 percent of FB total
  • Revenue per User: $4.32

Europe

  • Users: 272 million, or 23.5 percent of FB total
  • Revenue per User: $1.87

Asia

  • Users: 339 million, or 29.4 percent of FB total
  • Revenue per User: $0.75

Rest of World

  • Users: 346 million, or 30.0 percent of FB total
  • Revenue per User: $0.63

U.S./Canada is by far the most lucrative region by geography, with Europe an obvious second.

But consider that from the first quarter to the second quarter of 2013, Facebook grew its monthly active users less than 1 percent quarter-over-quarter, and year-over-year growth was a measly 6 percent.

In its second-quarter numbers, Facebook reported that $848 million in revenue — almost half of its $1.8 billion in total revenue on the quarter — came from users in the U.S. and Canada. So even a small rollback here is going to be felt, and the lack of future upside is significant.

The situation is the same in Europe as well. Europe's monthly active users increased just 1 percent from the first quarter to the second, and a modest 10 percent over the second quarter of 2012.

The Facebook longs better be sure that this user base is going to stick, or else they are in serious trouble.

Mobile and margins amplify the problem

The bulls may contend that the problem, then, isn't growing the Western audience, but simply monetizing it better. And, oh, by the way, if you make more money off those "rest of the world" subscribers, then it won't take quite so many of them to offset lost U.S. revenue if the users do roll back.

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Fair point. But Facebook is hardly in a great position to pull this off thanks to its big (and necessary) migration to mobile.

If you think it's easier to monetize on mobile devices vs. a desktop experience, then you haven't been paying attention to Internet stocks at all. Smaller screens categorically mean fewer advertising opportunities, lower rates and less user engagement with the content of those ads.

Want proof? Just look at where Facebook's ad money comes from. About 41 percent of ad revenue came from mobile last quarter, but 71 percent of its users were on mobile — 819 million users out of 1.15 billion total.

Similarly, a push into video may not be much of a savior for Facebook advertising even if it is a pivot from conventional display ads. Pre-roll video ads significantly reduce engagement with video across the Web, and for rates that may only be incrementally higher for Facebook when compared with current advertising options.

There are admittedly opportunities with Instagram advertising, but most analysts don't expect that money to materialize significantly until a few months into 2014.

And while the recent push with newsfeed ads shows that Facebook is willing to get creative with advertising in the meantime, the fact remains that Facebook is swimming upstream.

Facebook stock overheating

Michael Santoli recently reported "only twice in a generation — and probably only twice in history — has a company that started with a market value of at least $40 billion doubled within a single quarter, both times near the climax of the late-'90s tech-stock bubble."

Those stocks? Oracle (ORCL) and Yahoo (YHOO), which both quickly crashed as the dot-com bubble burst, and which both remain over 60 percent down from those highs.

That's quite a finding, and one to keep in mind.

I'm not saying that Facebook is going to get slashed in half overnight. But it's worth noting that after Wall Street breathed a sigh of relief in July to spark a big rally… the expert analysts out there set price targets that were met in a hurry.

Some of those targets were subsequently moved up, but right now the median price target on Facebook among 36 analysts surveyed by Thomson Reuters is $53.50 ... right about where Facebook stock currently sits.

And at a forward price-to-earnings ratio of over 50, there's clearly a big expectation for growth with no room for error.

Sure, institutional ownership will provide some stability; Fidelity Contrafund owns more than 1 percent of Facebook now, for example, and about 50 percent of shares are held by institutional or mutual fund managers.

And a big headline like an acquisition could certainly rejuvenate the bulls, as could nice growth in users that proves me to be a complete buffoon.

But in my opinion, the easy money has long been made in Facebook as it doubled from $25 or so to over $50.

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This upcoming earnings report may not signal the end of the road just yet for Facebook's roaring run from the doldrums in the middle of last year, but the momentum is bound to end soon given the pressure on European and U.S. user growth, and the margin pressures from mobile.

So if you're sitting on a pile of money in Facebook, consider taking some of that off the table sooner rather than later.

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