Worth searching, there are a few posts from people who tried this. Generally there are results, but they seem to not be as strong as people expected. Some people have had a credit or a trial that made it worth trying.
So short version - if you have a coupon or credit, why not. If you are paying completely out of pocket, you may not get a bang for your buck.
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.
I've personally spent over $7,000 of my own money on Facebook video promotions ads and continuous page promotion ads in the past 6 years and the results are mixed. It has been exhilarating to see thousands and thousands of views that I would not have garnered organically, however, it has not helped me build community on my Facebook ministry page. Part of that may be my fault, but I approached it from more of a brand awareness perspective than anything else.
Depends on what your goal is. If you are a new Youtuber, facebook ads is very good for brand awareness. I have used it over 2 years now but will be stopping this year once I hit my 10,000 subscriber. I have gotten subscribers through facebook ads and I will it does work and help break into different country as you can target your audience. Now, facebook ads only helps when the content is what the audience want. I have built a close to 10,000 facebook page and 10,000 facebook group but to be frank it doesn't bring in the views as my audience are mostly Africans and have internet issues. Be careful though, facebook ads rules is a book to study on it's own
I've not yet used Facebook ads to push videos, but in the past, I've had mixed results with Facebook ads. I would recommend using Google ads on YouTube since your potential audience would see the ad ON YouTube. Since they're already on YouTube, they're not really moving from one network to another to view your channel. I personally find that when I'm on Facebook, I want to stay on Facebook and when I'm on YouTube, I want to stay on YouTube. Results may vary. I'm one data point and probably a weird one at that!
I think facebook would be a great way to get some exposure but I don't know if the people you would get would build your audience. Unless your facebook is literally in sync with your channels topic, people who visit your channel from your facebook might more then likely just be curious rather then interested in your content. They wouldn't subscribe probably if they view the video since they wouldn't exactly be the audience your looking for.