Too big and too little
What's the point?
The company has been going so long (and grown so quickly) that its future expansion is quite limited - so not much scope for organic growth. Yet for all its size, it's only planning on raising $5Bn from the floatation.
That's enough to land it with all the regulatory deadweight that a public company must conform to, but not cash enough for it to drastically change its business model. All FB could hope to do would be to gobble up other companies in exchange for it's own shares - presuming they hold their value on the stock market - and then hope it can assimilate whatever novel attribute the small-fry had into it's own corporate mass. A strategy that's not known for its successes.
This IPO would make sense if the owners were planning to cash out and wanted to spend more time with their money. But in that case, without the founders' vision, who'd want to be left holding the shares?