The usage of a tool like this is an issue of convenience and it's more convenient if you have everything inside one single app you use regularly, multiple times per day. That's Twitter, at least for Twitter users. It's easier to do it all from one app than to bust out another app when you need to, that tiny tiny tiny fucking almost insignificant amount of effort between opening another app or using the one you always use all day anyway is enough to create usage.

You don't need to do something unique or invigorating, you just need to be the most convenient for users, people will use whatever takes the fewest buttons to carry out the transaction.