Even if the sale price of the company was really $2B, they were able to take Facebook stock that is on the verge of an IPO for 75% of what it will IPO at. If Facebook hits a market cap of $150B in the next 12 months, that would put the value of the Instagram stake at $1.4B a year from now, plus the $300mm in cash that Facebook chipped in.
Looking at the deal that way, it made a lot of sense to do it now if Instagram was shooting for a $2B exit.