I can tell you why I do it.
An active Facebook page affects where you show up on a Google search. Some types of businesses are still rated by classic SEO (search engine optimization), but more and more, Google is going to directories.
For example, if you search the common keywords for my business (pet emergency, emergency vet, etc), we pop up first in the feed, and our Facebook page pops up even higher than our website.
Facebook is making more and more money on internal advertising (which is what it was designed for, after all – to make money), so I’m sure that there will be more and more of it. I don’t actually use paid advertising (at least, not yet), I use our Facebook page to develop community goodwill, to educate, to inform, and to entertain.
It also allows me some leeway in what nonprofits I can support. We are just like everyone else, we have a budget for financial support for worthy causes, but we always wish we could do more. If I link a “fan drive” to some cause (as I have going on right now ), I get to do even more because I’m technically using part of the marketing budget as a donation.