That's precisely the point. Silicon Valley is saturated by a bazillion products, so the audience here is extremely discerning. Most people will try most things, but to gain critical mass like that within Silicon Valley is very, very difficult.
Empirically, there's a very good chance that a product with critical mass here will tend to spread like wild fire to the rest of the world (and I suspect Peter Thiel has much better data on that than you do). Since the purpose of investment is to buy low and sell high (and the purpose of early stage investment is to buy very low and sell very high), putting money into Quora is as good an early bet as one could hope for.
I disagree tho that doesn't mean Quora won't be successful. It's just not realistic to think SV is representative of most populations. It's a very tech-heavy population which is not necessarily what the masses are looking for. Quora could be a dominant Q&A site for tech issues... and perhaps that's worth the big money (I just doubt it).
Empirically, there's a very good chance that a product with critical mass here will tend to spread like wild fire to the rest of the world (and I suspect Peter Thiel has much better data on that than you do). Since the purpose of investment is to buy low and sell high (and the purpose of early stage investment is to buy very low and sell very high), putting money into Quora is as good an early bet as one could hope for.