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.
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.
They seem to be running from one political crisis to another, so I guess they need some political savvy.
Plus most of the UK despise him.
He is known as an unprincipled opportunist, he will fit right in at Facebook.