So imagine one of those other bidders had won, at a lesser price. Facebook might still have taken a 5% hit, facing a newly-empowered competitor. At least now, as with Instagram, the threat has been absorbed into the mothership.
It's a rich price, but arguably strategically justified.
The people who criticized the prices paid for YouTube and Instagram were very wrong. Also wrong were those who said Facebook was crazy for not accepting Yahoo's buyout offer, or that private investors were crazy for giving Facebook its rich pre-IPO valuations. Facebook keep chugging along, knowing and confident in its understanding of the opportunities and threats.