From my (admittedly very limited) experience, it really pays off to take your time and try to get the details right. Nothing that I say below is the objectively "right" way to go about things, but this is what's been working best for me to date:
Location: All major English-speaking countries, a few other European countries, perhaps a couple countries in Latin America or Asia. I generally avoid Africa - while it yields many cheap likes, there is an increased risk of getting liked by bots, which are then useless.
Interests: These are hugely important. It doesn't matter if people see your ad if they don't give a toss about it. Depending on who you want to attract at the time, there seem to be two obvious routes. If you're looking for genuine supporters, some good categories are Flat Earth (duh), alternative medicine, conspiracy theory, moon landing conspiracy theory, religion, etc. - you know, the kind of people who are likely to question scientific authorities. Alternatively, you may want to look for people who will follow us because they find us bizarre - in that case, the obvious categories would be science, pseudoscience, US Politics: Very Liberal (hello, r/atheism), etc.
Finally, the "creative" bit. When I first started with this, I assumed that nobody will care about what picture you slap on an ad. If they care about the Flat Earth Society, they'll look into it, right? I'm not gonna lie, it still baffles me a bit, but choosing the right picture is much more important than I would intuitively think. Perhaps this is just me being me. Anyway, I ran a quick test just now. I set up an advert set with a budget of $20 and created 5 identical ads, differing only in the image used. The actual ad text I used was somewhat uninspired ("What is the shape of the Earth? The answer might surprise you!") and it probably makes my ad worse than something I would normally put out. Here are the 5 variants of the ad I used for this test:
The page's cover photo - seems like a sensible enough choice. Blanko's design is snazzy, and it makes the whole thing look professional. Right now, it's performing at $0.13 per like.
A fairly generic stock photo I grabbed from Facebook's picture bank - not particularly relevant, and not particularly effective. $0.28 per like right now.
An irrelevant map - not in line with FET, and while it looks pretty cool, it obviously doesn't draw people in. $0.39 per like.
The horizon - abysmal. I can only imagine that those who aren't already sympathetic towards FET in some way look at it and think "hey, that looks round to me". $0.78 per like. I'm turning this money sink off now.
A pretty Flat Earth map - this one takes the cake. At $0.10 per like, I think this one could get us quite far if I actually worked on tweaking the ad further. Will definitely use it in future, more serious, work.
On top of all that, Facebook has recently introduced the option of video ads. I was thinking about experimenting with our homepage header video and turning that into a Facebook ad. I imagine some extra motion could be quite attractive. Someone with a better understanding of this stuff could also probably capitalise on making gender-specific ads. It looks like Blanko's header image appeals to women more than it does to men, while the "pretty map" is much more gender-balanced.
Together, this ad set has so far generated 80 likes for $12.59, averaging at $0.16 per like. Since Facebook's algorithms rely on a feedback loop, the best-performing ad has been prioritised (displayed 535 times, yielded 50 likes), while the worst-performing one has been slowed down (277 views, only 2 likes). I now have an option of completely killing off the strains that don't work and letting the ones that do work make good use of the remaining $7.41. I will be doing just that, hopefully securing another 80 or so likes.
I hope I'm not making myself sound like an arrogant know-it-all here. I also apologise if this is all obvious to others. A lot of this stuff took me a long time to get my head around, since it goes completely against my intuition. So, in the interest of saving others some time, I thought it might be worth documenting.