What is the difference between paying per click and impressions? Advantages/disadvantages?
Cost per click means that you only pay for each time someone actually clicks your ad. So you get to show it to a ton of people but only pay when people actually click it. Basically this is what you do if people aren't very responsive to the ad, and is generally a safer route to go. But your ad might get less rotation if people arent clicking it because FB wants to generate revenue.
BUT
if people are clicking your ad like crazy, then its better to do cost per impressions. Mainly because itll be cheaper. Basically if you have a high click rate, it means your paying each and every time someone clicks it, so it gets expensive fast, or you run out of funds fast. but if youre only paying for impressions that means it doesnt matter how many people click it, because youre only paying facebook to show it. also your ad doesnt get moved out of rotation because facebook is generating revenue for just showing the ad, and not for the clicks.
ideally, what you want to do is to get as many clicks per impression as possible. the best thing to do is to start off with CPC and if you get over 2% after 10 clicks, switch to impressions.
also, it depends what your ad motivation is. if you're just doing branding/association [i.e spreading knowledge, so people just "know" the name ron paul & associate it with anti war or other catch phrases] then its better to do impressions.