It is absolutely about growth and user acquisition. They are using their capital to subsidize and lock in a monstrous market. Is that bad? There are reasonable arguments on both sides.
But Facebook is not a charity. Facebook is not a social cause.
If it were, Zuckerberg would have sold his shares and reinvested the cash back into Facebook when announcing his plan to donate $45bn. There's a reason billionaires invest their money in charitable funds, and not all of it for tax reasons. There are some things markets just aren't great at solving.