I should really leave this one alone and let you guys answer it.
Really I should.
1) You can't use any social media and NOT see an advertisement ...to advertise.Look for the unmissable button/phrase/tab/advert/banner/link - thing - you have to REMOVE (x) to use your social media - that says PROMOTE.
Click that button and it will easily answer all your questions.
2) No, can't do that? Okay. Generally, this is how they all work. Choose to advertise and then pick a target audience. Pick some fields of interest or #tags, if you will. Choose the amount you want to spend, over how long. You can a long campaign or a one-shot short one. Spend as little as $20 bucks or $200 etc....
3) Make your ad, it could be a picture, a video, slideshow....a bunch of stuff and they have templates.. You can link to a store or link to a LIKE button or subscribe button or follow or whatever.
All your other questions need consultation. It makes no sense to try and give you "business advice" without seeing/knowing your business. Before we get confused, this is business advice since you mentioned actually spending money.
I think before you worry about effectiveness, which would be CRAZY for someone to insert an anecdote here because no two properties, campaigns or outcomes can truly be compared or judged in a vacuum.
You probably need to evaluate WHAT you are after and how easy it would be to garner those results/ attention from ads.