It makes perfect sense to me.

While farmville is a bullshit game, it's one of those endless things that can pretty much suck anyone into. It really spans generations and clique archetypes because of it's ease of gameplay, goal, and availability. I mean who doesn't have a computer or a facebook account today?

Plus, as strange as it sounds, it provides some kind of sense of community, where people have to help each other to go and make their farms better, by feeding animals, trading and all that.

From what I hear, once you start it, it's pretty addicting and hard to stop.

Now, let's look at Final Fantasy.

Yes, these games are GOOD (5, 8 and 10 aside). That goes without saying. However, they are often complex and bring many factors into play. People are stupid, they like simple.

Also, not everyone has the time to go and grind out these 100+ hour games, especially when you're working 9-5 and have social lives outside of work. Factor in girlfriends, kids, etc and that time gets cut even moreso. That makes these lengthy games with complex stories harder to follow, especially if you're playing them a few times a week for maybe an hour or two at a time. Farmville is just a couple of clicks to maintain things, which you can do while at work, and that's that. It's not a deep game, and you really can't forget any complex subplots. Oh shit, where is my ugly duck?!

Also Farmville is on facebook and is like THE cool thing right now. Final Fantasy has established itself as a gamer's series, as already has been stated. Most casual games are familiar with 7, because that had an insane marketing blitz, and had the allure of being the first FF with 3D (ish) graphics and the first one on the PS.

Long story short, Farmville is the more popular game over FF simply because of it's simplicity, ease of accessibility, status as THE popular game, free cost and the super sneaky hook of getting you casually addicted.

FF, while it is clearly superior in terms of story, gameplay, graphics and all of that shit that people really value in games, just can't compete with the addictive nature of Farmville, nor will it ever have that epic reach, mostly because of the niche it's carved for itself in 20+ years.