i think the site is financially powerful enough to operate without external advertisers though, which is i imagine part of why they put the promoted posts on their own page - since the site is going through a 'proof of concept' phase, they maybe wanted to show the site can prosper even without serious paid adverts.
from my perspective, i am just thinking currently of how to get more eyeballs on my posts and it was claimed that promoting the post is a way to do that - but so far, as somewhat expected, it has not really been very effective. i'm not sure how long the promotion even lasts!