The entire concept of Twitter still makes no sense regardless of how many people are using it. It's coasting on momentum at this point and might give out any second.
Their core strength is on the Individual -> Broadcast communication angle. Facebook tried to pick it up, but their mental model is still firmly Individual -> Group & Individual -> Individual.
Twitter's "Follow" function being the core method of interaction, and defaulting to public feeds that you can easily interact with is actually fairly different even from a blog or say tumblr, whose core method of interacting is content-first. The balance of power (you can tweet, I can tweet back, I can retweet you, you can retweet me—and they all look the same and have the same weight in the UI) is the main advantage, and if they can play that well then they have a chance.
Twitter's "Follow" function being the core method of interaction, and defaulting to public feeds that you can easily interact with is actually fairly different even from a blog or say tumblr, whose core method of interacting is content-first. The balance of power (you can tweet, I can tweet back, I can retweet you, you can retweet me—and they all look the same and have the same weight in the UI) is the main advantage, and if they can play that well then they have a chance.