1) Tweets disappear quickly from a user's timeline 2) Most people have few active followers
1 and 2 work against each other to form a negative feedback loop. The result is the vast majority of users use it passively to read celebrity tweets, nothing more.
Twitter must know this as their growth is already peaking and they're not making much money. Expect to see more drastic measures to try and turn what they already have into something profitable. They've peaked.