I would choose Facebook simply for name recognition. Having Facebook on your resume would carry more weight with many potential employers and partners.
This is like telling someone 3-4 years ago to work at Google instead of Facebook. The people who joined Facebook then, before the brand carried any weight, came out way ahead.
Well, yeah, but it's also like telling someone 3-4 years ago to work at Google instead of Digg. The people who worked at Digg, now that the brand doesn't carry much weight, ain't doing so hot.
The problem with betting on an unknown is that you're betting. The returns can be high, but so are the risks. If you're comfortable with those risks, it can be very lucrative. But it can just as easily go south.
