Outbound clicks measure the number of clicks on ads that take people to properties away from Facebook. This is different from the link clicks metric in that a link click includes clicks to properties on Facebook (like instant experience, lead ad forms, and collections).
Outbound clicks may be superior to link clicks in regards to measuring the ability of an ad to drive traffic away from Facebook, though landing page views would also include those who clicked the link and allowed the landing page to load.
Some related reading:
