I feel pretty sure we'd have been impressed enough by Max and Peter to fund them regardless of the idea (which initially had almost nothing in common with Paypal).
I knew Ev before Twitter so I'm sure we would have funded that.
Pandora I know nothing about, so I can't guess there.
SalesForce I'm pretty sure we would have funded because Benioff radiates "winner" in much the same overwhelming way that Zuckerberg does.
Instagram is the one we'd most likely have missed. It all depends when we'd talked to them. They were a kind of overnight success in traffic. If we'd talked to them even a day after they launched we would certainly have said yes. But before that it might have seemed too speculative.
I don't know about FourSquare. I've never met the founders and don't understand the business.
Pinterest we definitely would have funded, because Ben is a two time YC alum (with different companies alas). We knew he was good because the first time he was part of a startup that as an experiment we didn't make move to California. As Demo Day approached, they were in terrible shape. But Ben swooped in at the last moment and gave one of the most convincing Demo Day presentations.