It must be frustrating to spend time promoting your Facebook group only to have your Craigslist posts flagged and removed.
At the same time, please consider the people who use the Free section for its intended purpose. Many are genuinely trying to give away items they no longer need, while others are looking for things they can't otherwise afford. Filling the section with unrelated or fake listings makes it harder for those people to connect.
If you want to advertise your group, please use a category that's meant for promotions or community groups instead of the Free section. It would help keep Craigslist useful for everyone.