That's the straightforward reason, but it doesn't sound big enough to me.

First, AirBnB just got valued at 10bn. FB's IPO broke valuations records. Reporting requirements are a drag and I'm sure they impact IPOs on the margin, but 10bn is not marginal. Reporting for a company like AirBnB is not that hard. They have a straightforward business model with one business. It's not like they have 50 years of hair, cross ownership, JVs and zombie businesses.

Second, FBs timing indicates (to me) that something else is going on.

Third, WTF is the reason for these arbitragers. Arbitrage requires a willing seller. Why raise capital (sell shares) at a lower cap 9 months before an expected IPO?