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.
1000-2000 users and a $15 million valuation? That seems pretty overpriced to me: even if you could see a positive future, FB's success outside the Ivy league crowd was no means guaranteed.
It's worth noting that Peter Thiel later put in $500,000 for 10% of the company, which would value FB at around $5 million, and that was a few months later.
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.