I think both Google and Facebook can be great optiions for paid advertising to promote your forum. Both are powerful platforms with a lot of potential users. What matters most is that you create an effective ad and define a set of goals to measure its success. That said, I find that Google tends to be slightly more useful for driving traffic at scale, while Facebook is more effective for engaging with a specific audience. I suggest that you research each option in-depth, including familiarizing yourself with their unique features, targeting options and pricing models. Ultimately, go with the platform that best meets your needs and budget.