Is it better to target a small group of people with an appealing, tailored, custom made message? Or targeting a large group with a generic message?
If you are up to date with the modern marketing techniques, you might say it is the former. Or if you are still in the industrial age, the second option sounds more exciting.
Neither is true. You need to find your optimal audience size.
If you want to reach the entire US population on Facebook, you are competing with anyone in the US (and in the world!) that has a larger budget than you. And Facebook will favor (i.e., serve ads) of those with the biggest budget (not you!.)
If your audience is too small, even if you have a decent budget, you won’t reach them all, their Facebook usage will be low compared to a larger group, and you won’t get the volume of results you want.
What your in-house or third-party ad tool should tell you is what’s the optimal audience you need for the budget you have.
If you have a $30 daily budget, a 3,000 people audience might be just enough, if you are expecting 1-5 conversions per week.