My 21 and 23 year old daughters have moved back in with us. They stay up until the wee hours of the morning watching streaming media, then wake up mid afternoon to get on social media, eat dinner with us and then repeat. I can't wait till they leave. At least when they are living on their own I don't have to know about their bad habits.
I agree that if they're living with you, they should probably be doing chores, cooking, and getting some exercise in. But if they're financially constrained and have nothing to do, futzing about on the Internet and social media isn't a particularly bad use of time (even if it's also not a particularly good one).