Large margin between Facebook Ad Click and Piwik Stats

At the moment we are running Facebook Ads with like to a webpage. However we realized a large margin between the clicks the Facebook ad manager display and the click on the webpage from Facebook.

Facebook: 338 clicks
Piwik: 79 clicks

See attached file - areas with the red dot.

Anybody have a clue why Piwik does not count that many referes from Facebooks?

Thanks.