I think I probably would've passed on giving The Facebook money at a $15 million valuation during that first year. The article notes that the user base was in the small thousands, so not so much evidence that it had any traction. And there were certainly competitors as well (some of whom actually had a better feature set). Not to mention the whole uncertainty of deriving revenue from the site, an issue Facebook's still dealing with today.

edit: I missed the latter part of the article. It indicates that Thiel chipped in in half of Zuckerberg's "minimum," for well short of his proposed valuation to Battery.