Sounds like an odd decision, but Nick Clegg is a very level-headed person with lots of experience handling tricky communications in party politics, elections and government policy, so potentially a good hire.
Plus most of the UK despise him.
He is known as an unprincipled opportunist, he will fit right in at Facebook.
They seem to be running from one political crisis to another, so I guess they need some political savvy.
Lots of experience - but arguably proven incompetence, also. He successfully established himself (and his party) as the scapegoat for a broadly unpopular austerity campaign, while gaining no lasting concessions in exchange. His tenure essentially wiped out the Lib Dems as a functioning party - dropping them from a height of 57 seats, to just 12.