Once again you are going into it to make a profit with that mindset. You asked why would someone want to start a large facebook group. You don't actually care about that, at all. You care that not only is facebook taking your revenue stream, that its enabling people who have absolutely no interest in your revenue stream to eat up small chunks of the market. Look at all the people who come to presales and have no idea how a domain works. If their interest is served by a group, facebook just solved all of their problems. They aren't going to care where the revenue is going if 100% of their interest is served already. It didn't start out about money and when that is the case it usually only becomes about money when the hosting costs become a problem. Which no matter how big the group gets, there are no costs.When you start a forum your future is in your hands. You can decide to offer a paid membership, use Google Adsense, sell banner ads, etc. When you start a Facebook group you can create something "big", but it's under Mark Zuckerburg's umbrella. Only Zuckerburg, his employees, and shareholders profit from the effort of FB Group's Admins.
Personally I hate everything about advertising. 99% of the time you can't sell me anything. I walk into a store and when someone asks me if they can help me I start rattling off part numbers to see how they will react. If I want to know a product exists I will read about it. The most effective advertising I have ever seen is for things I already bought. Which isn't very effective. It's a huge waste of the advertisers money. If a site can offer paid features that make a profit without limiting the user for no reason then I totally support that. It works well for some sites it doesn't work well for all of them.