Whether you use the term "uncommon" or "not popular", they're both relative. Uncommon compared to what? Less popular than what? The poster answered those questions by mentioning the AR-15 and AK-47. I would generally agree that you see a LOT more AR's and AK's than you do FAL's. Why? Several reasons. The AR and AK, of course, use intermediate cartridges, so they are often easier to shoot. The FAL and other "main battle rifles" like the AR-10, G3/HK91 and M14/M1A use full power cartridges that are more expensive and much harder on the shoulder and ears over long sessions. Thus, they're less fun for plinking. The main battle rifles also tend to be heavier and more expensive than the AR-15 and AK-47.
I suspect that the most important factor, however, is media exposure. Because the AR-15 looks like the M16, which we all know has been the U.S. military's primary rifle for 40 years now, and because lots of police departments use it, too, you see AR-15's or M16's all over the TV and movies. People want what they see and hear about. The same goes for AK-47 for similar reasons. It is a legendary rifle that you see in ALL the movies, and the morons on TV never miss a chance to bash the evil, all-powerful AK-47. Again, people want they see and hear about, so there has always been a lot of interest in the AK (how ironic -- the more the media scream about the "evil" AK-47, the more demand for them they create!). And the AR-15 and AK complement each other nicely. Whether it was the Cold War of Russia/Warsaw Pact vs. USA/NATO, or the modern world of the US vs Islamofascist terrorism, the principal weapons are the same -- the AR-15/M16 on the side of the west and freedom, and the AK-47 on the side of our enemies. People like simple stories with clear good guys and bad guys, and you don't get much more "good" vs. "bad" in popular culture than the M16 vs. the AK-47.
As far as main battle rifles go, however, the FAL is VERY popular and reasonably common. It may not be quite as common as the G3/HK91/CETME/PTR-91 or the M14/M1A, but I still see a lot of them. In fact, I see a lot more of them than I do AR-10's. I have a FAL (DSA SA58 Para Congo) and I would never part with it.
