To like a status, comment, photo, video, or link on the social network Facebook. Can also be used to describe someone who likes their own status, comment, link, etc......
Example 1- Emilia gave me a Facebook Highfive for the link I posted about our friend's epic fail today.
Example 2- Dude?!?! You like everything on Facebook, INCLUDING your OWN stuff....... its like giving yourself a highfive..... a FACEBOOK highfive that is!!!