4) You build a win-big type company that is outside Facebook's area-of-expertise but leverages the advantage the Facebook's platform give you.
Takes games for example. Facebook itself doesn't seem interested in writing games for it's platform. Yes, it's true they want you to use their in-game currency, but that's just a platform tax, not competition against your business.
I suspect location-based-services are similar. I doubt Facebook wants to compete with the game-based mechanics Four Square is building. There is still a question as to if Facebook will go after the coupon market, though.
There are also businesses where you may be able to build traction on the Facebook platform, and then pivot off it if required.
Assuming that your (4) really is something other than my (2) or a subset of my (2): how big do you think the opportunity of (4)s is, realistically, over the next 5-10 years, from an investor's perspective?
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.