I’m just quoting an extract from Natalie Burg’s article:
Of those 100 or so fully composed daily stories, only about 15 will make it to the blog. The curators gnaw the contender list down by testing each with unpublished paid posts on Facebook. Rather than simply posting a link to their audience, PetFlow pays to distribute them to a certain segment of Facebook users who aren’t fans. If a post meets a certain criteria—a classified combo of likes, clicks, shares, or comments—it graduates to the blog and is shared with the PetFlow Facebook pack.
A fascinating look at the Upworthy-like curation system!