Maybe it's the same as your (2), or maybe Facebook makes more than a small amount from it.
In any case, I think it's a mistake that you dismissed this as The only problem is, such unique, exclusive opportunities are rare. Big winning companies are always rare, and there are always a lot more ways for them to fail than them to succeed.
I think it would be a big mistake to ignore the opportunities here and shun the unfathomable risks and complexities. There are always risks and complexities, and any kind of partnership increases the complexity. That shouldn't be something people should run away from, though.
How big do I think it is over the next 5-10 years, from an investor's perspective?
I think it could be huge. Zynga seems to be doing pretty well, as do all the other social app vendors (Slide etc). But there are a lot more opportunities than Zynga have covered, both in the gaming as well as the non gaming market.