Yes, old acquaintances posting about minutia of there lives is of no value and can be down right irritating.
But the single most important function facebook provides is it turns acquaintances into friends. You can add people on Facebook that you've only meet once or twice before. You don't need to awkwardly ask for someone number or email or on the spot make future plans to meet up. You just look them up and you're instantly connected to them. You may learn they have a hobby in common with you or that they have an event that you're welcome to come to, and through the connection provided by Facebook you can strengthen your relations with people you find interesting.
For my real friends that I keep in touch with regularly facebook is more or less irrelevant - in fact most aren't even on my facebook. It's a nice platform to upload your photos to I guess, but honestly emailing a dropbox link is kinda nicer tbh.