I'v used facebook ads a few times,
I tend to target specific areas and specific people / age range / interests etc
instead of doing an add targeted at everyone in the UK, I'll focus on people that would be interested in the product / service Im offering, and I'll target specific areas at a time so that for say two weeks, Im advertising to students in Liverpool for example, then I'll move on to another area and work my way round.
Creating an ad that grabs peoples attention can be quite tricky, an ad displaying a single product seemed to work better for me than just an ad advertising a store or a website and I tend to start with using the pay per click option instead of the pay per impressions, if your ad doesnt do well then it isnt costing you anything really, and if the ad starts generating a good load of click throughs, just change it to pay per impressions to save some money.
Before you create an ad, look at the rest of the ads displayed on facebook and try and figure out why they work, why they grab your attention and why they dont, then use that info to create your own ad
