Promoted posts are like the budget version of Facebook ads. With the right campaign, Facebook ads can do wonders, and for a couple of bucks the promoted feature really did more than I exptected (which I went into this thinking nothing would happen).
For your first point though, we actually made a point not to mention that free drinks or an open bar was involved. It just had a quote about drinking from a notable person, but nothing about us picking up the tab (we actually were not sure at that point if we could). Second, we are in DC, getting very close to election time, there is an over abundance of people who care about politics here. While I do agree that this is clearly not a standard experiment due to that lack of a proper control, this is a simple first hand impression of how much $5 provided us (which I hope I made clear through the post).
Between my research with going against TOS and attempting to purchase followers, and using black hat methods to increase your numbers, this is a really cheap and effective alternative that Facebook clearly does not mind us using.