AFAIK with facebook ads you pay for impressions, which may or may not turn into clicks, which may or may not turn into installs.
There's 3rd party services which sell likes / installs... but this are against the Facebook terms of service.
This actually creates huge issues for legitimate advertisers though!
Facebook will try to ban people who sell 'likes' (as pictured by Code Fox), so in order for these people to avoid detection, they have to 'like' a lot of random things, as well as the things they're being paid specifically to like.
This means that when you pay Facebook for a legitimate advertisement, there's a huge chance that you'll get clicked on by these illegal like-sellers, which is useless to you, as they're just fake accounts run by bots / sweatshops... So even with legitimate facebook ads, you often end up just buying fake 'likes'.
These fake likes are then toxic for your page. Whenever you post something on a page, facebook doesn't automatically send it out to all your followers -- they do it batch by batch to see if it's actually engaging. If you've got a lot of fake followers, they won't engage with your posts, so your post will stop being propagated once it hits a batch of fake accounts...
TL;DR Facebook advertising is very broken.