Twitter's user growth is tapering off alarmingly quickly. With an active user base of now 255 million monthly active users, the apparent deceleration of Twitter’s growth suggests that the service will have a very hard time coming close to Facebook’s user base, let alone matching it. When Facebook's growth rate first dropped to the level where Twitter's currently is (25%), Facebook already had 800 million users. When Mark Zuckerberg was asked why his company had acquired WhatsApp for $19 billion, he replied that services whith the potential to reach a billion users are hard to come by. It seems like Twitter isn't one of them.
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