The difference between Twitter and Facebook is that Facebook can make money.
In case of Facebook, it would be like:
"Can you get more users?"
"Nope, we already have the whole planet. However, we can squeeze more money from them. The users already accept that their timeline is created by an arbitrary algorithm. So we can make them bid against each other for appearing in their friends' timelines. If you want your mom to see your photo from holidays, you can pay $1 to make sure it happens."
In case of Twitter, it would be like:
"We already have the whole planet. We can ruin people's careers, or drive them to suicide, more than any other web service."
"Great, but can you make some money? For the shareholders, you know."
"Uhm... if we started charging people for using our service, most of them would leave, so..."
"In that case, no more money from me."