People in their 30s and 40s earn more money than college students and new grads in their 20s, and they are spending that money right now. Facebook groups for home decorating, design, cars, fitness, parenting, all of those are really popular. You can go on Facebook and find a large community dedicated to literally any style of home decor you can imagine, and those people are quite willing to spend money.

Same with the parenting groups. DINKs don't spend money at anything near the rate parents do. DINKs have higher free cash flow, but that is because they have free cash, parents are buying stuff all the time.

Honestly, 5 years ago I barely used Facebook, now that I am older I am using it a lot more.

Also, with that entire population decline thing, each new generation is, at best, the same size as the generation that came before it. If constant double digit growth is desired, sure, need to get that next generation on board, but at some point Facebook may have to be happy with "just" earning hard to imagine amounts of money each year and only seeing single digit growth in revenue.