I'd like to add to this.
. There is also virtually no chance of data leakage with phpBB provided all software components (PHP, MySQL, phpMyAdmin, Apache/Nginx/etc.) are kept up to date. phpBB has gone through at least one security audit and a lot of security testing, including patches, by the development team over the years. With Facebook, you are literally the product, member or not. Everything you type and everything you upload (photos, videos, documents, etc.) becomes Facebook's property the moment you use their service. You completely give up your rights to privacy and ownership of your data since you are not actually paying any money for the service. Each member is worth approximately $12 to Facebook with about 1 billion users to date and growing. It is proprietary software that uses complex algorithms to identify your interests and needs. You don't know where they sell your data to, the extent of it, or what else they do with it. For phpBB, your data is not sold to corporations for the purpose of advertising and profiting. Even though phpBB is not as popular as it once was, the software has gotten better as time passes thanks to all of the volunteers and contributors who add/edit code. They do this not because it is their job, but because they want to without financial profit or to find new ways to take advantage of people like Facebook does. Facebook relies on loopholes and legal exploits with addictive elements added to their site to keep people browsing, thus being exposed to more advertisements and tracking where you visit, what you've seen, what you've liked/disliked, which ads caught your attention, what your browsing habits are outside of Facebook, and so on. They then throw new changes in their long ToS list which you have to agree with when you sign up or log in. If you love ice cream, they know which flavour you prefer and you will get advertisements for ice cream. If you have dental issues, they know that, too, and will advertisement dentists to you in your local area. If you have a certain chronic disease, browsing the Internet with a Facebook cookie on your system will send them that information and it will be stored in their database, and you will get advertisements for that particular chronic disease because that is what makes them make money. They know more about you than you know about yourself. The Facebook website looks innocent enough. It runs well and is very interesting. But there is a big cost to using it through the use of exploitation, manipulation, addictive elements, giving up ownership of your files and information when you upload literally anything to the site, not respecting original content made by authors (e.g. videos) by allowing anyone to re-upload them to facebook on your profile without credit or monitory transfer, getting advertised to, etc. They are the business and you are their product. They will make money off of you through any legal means possible. If any of this makes you feel uncomfortable or concerned at all, that's probably a good thing.
Feel free to correct me if anything I've said happens to be untrue.