Thumbs-up on Facebook (4)
I believe the answer is:
like
'thumbs-up on facebook' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for like that I've seen before include "a match?" , "Find appealing" , "Relish" , "Care for - much the same" , "Such as; find agreeable" .)