It's in the article (see google cache). They were about to be valued at $500M in the next round of investment, during the next week. Zuck had to move fast, and

It's in the article (see google cache). They were about to be valued at $500M in the next round of investment, during the next week. Zuck had to move fast, and he did. He bought them 2 times the supposed valuation, which is probably what Instagram expected. (PS: Negotiation 101, start with a number 2x bigger than what you expect)