Zynga needs Facebook. My wife would have never started playing any of Zynga's games if there were not on Facebook, and neither would most of those whom I know play these games (my mom, sisters, etc.). In fact, I expect almost all of them would _stop_ playing these games if they were taken off of Facebook.
Online games much, much better than Farmville and Cityville existed for a long time before Zynga came around and none of these people played online games. Now, since it's integrated with Facebook (meaning it's easy to get to and play constantly, and has a good introduction vector) and since you can do nice little things like help your friends, Facebook denziens eat it up. It's weird.
I think the parent question was more thinking long term, at what point will the opportunity costs of focusing so heavily on Facebook outweigh the benefits to Zynga?
I can think of a few situations where this is possible:
1) End of World scenario where Facebook ceases to engage users
2) Zynga has collected a critical mass of email addresses and could successfully drive them into a proprietary wholly-owned portal if they wanted to
3) Facebook creates a conflict of interest e.g. Facebook-branded games, that are awesome
4) Slowly, revenue from non-Facebook channels outgrows that from Facebook or non-Facebook customer acquisition becomes far more cost-effective etc.
Any one of these scenarios could play out. As could dozens of others - it's going to be interesting to see if the Facebook/Zynga partnership lasts or if Zynga makes a grab for independence at some point, and why.
The only scenario that seems realistic to me is that somehow Facebook gets offlined or seriously depleted. It might be hard to think of ways that could happen, but it could happen.
I find it really unlikely that Zynga will be able to replicate its success on Facebook with a Zynga-controlled Facebook clone (maybe possible in case Facebook dissolves, as above) or a mailing list. Such tactics do not generally succeed for others who try them, and especially those that exploit email addresses are despised and ignored. My family would ignore any spam message to come play a Flash game, and I would probably advise them to do so. But they don't ignore games integrated into Facebook.
Facebook Games may aggravate Zynga, but I really don't think they could do anything about it. Without Facebook, they would lose most of their US-based users.
I suppose something could come that would surpass Facebook in terms of lead generation, but I have a hard time thinking of anything that would realistically do so in anything resembling the near future.
Facebook is different from MySpace, LiveJournal, et al. It's lock-in and reach is much more pervasive. Did your parents and grandparents have LiveJournal or MySpace accounts? Facebook has accessed and locked in an audience much more extensive than any of the internet social networking fad sites before it. And, as we see with Microsoft, once you reach a certain level of pervasiveness among a certain market, it is almost impossible to break that lock-in.
I have tried to understand (and annoyed my wife significantly in doing so) why people, women in particular, are so much more apt to try things if they are framed with Facebook's layout and have some superficial integration, like sharing an item with a friend, than if they were just normal games out there on the web. I have tried to understand why these people would stop playing Farmville just because it disappeared from Facebook and moved to another web site. I can't really understand it. These people act like there is no internet outside of Facebook.
5) Facebook's terms and conditions have adverse effects on Zynga, enough to turn them away from the platform.
Zynga is only going to stick around if there's money to be made. If there's 1 billion users but only marginal profit to be harvested, Facebook's lock-in and reach means squat to Zynga.
It's entirely possible that Facebook's terms and conditions will evolve to somehow disrupt Zynga's revenue stream (e.g. no forms of game currency other than Facebook credits).
There's so many ways it could happen it would be prudent for Zynga to have a plan B at the ready. I'm sure they have one.
Online games much, much better than Farmville and Cityville existed for a long time before Zynga came around and none of these people played online games. Now, since it's integrated with Facebook (meaning it's easy to get to and play constantly, and has a good introduction vector) and since you can do nice little things like help your friends, Facebook denziens eat it up. It's weird.