Seems purchasing Facebook promotion is probably through Facebook rather than TubeBuddy, I've moved this to YouTube Strategies. I'm not a fan of FB so I can't really speak to the effectiveness of it, and at the end of the day subs aren't as important as watch time. Using FB promotion as a way to speak awareness of your brand could potentially work. I have friends who've used promotion ads to promote their nightclub livestreams that they do on twitch and they get pretty good numbers, but I'm not sure if that's because of the ads, or because they already have an active presence on social media and people already knew about them.
At the end of the day as a brand awareness strategy, sure, but just to grow subs I think you'll be disappointed.