Great questions and some great responses below. Some information that I thought could be helpful:
Running Ads on Facebook/InstagramTypically we let Facebook's algorithm decide where to post ads. Its optimization will be very fast for a short-term job post. However, most people aren't thinking of "applying for a job" when on Instagram. That platform works better with branding.
Boosting a PostFacebook makes it very simple to "boost" a post, but I would be hesitant to do that. Boosting a post has limited targeting capabilities. Also, a "Boost" is designed more for engagement (likes/reactions), which might not line up with your objective for collecting applications. It can work for being a quick boost on a Facebook job, but I wouldn't post content on your timeline and boost the timeline content.
Using Facebook Business ManagerThe Facebook Business Manager provides more robust options with objectives and targeting.
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