Having 250 million members, if Facebook was a country it would be the fourth country in the world measured on population size (US is third with 300 million, India and China are the numbers 2 and 1 with more than one billion people).
With 250 million active members, one out of approximately every six people who can be online is a member of Facebook.
Why should Facebook want to leave the traditional web? Because they can? Facebook has an immense userbase and this userbase contributes immense numbers of data (which can be information and knowledge) to the platform. Every day five billion minutes are spend on Facebook, every week one billion pieces of content are added and shared. Most of this content is not shared with the outside world, is not indexable by for example Google and is thus only available within the great wall of Facebook. So Facebook already left the Web by keeping most if its content inside its network.
Facebook does not need other sites on the Web, other sites need Facebook. Why would Facebook still hinder its members with the rather old-fashioned WorldWideWeb (which is slow, since you first have to resolve the URL into an IP address via DNS, then do the HTTP request to the web server, than receive the HTML and other files and than render the page) and why wouldn’t they introduce their own implementation (facebook:// instead of http://) which could do more than just the aforementioned process?
Facebook has the size to introduce its own browser-like platform, its own operating systems and perhaps even its own hardware line.
Facebook could disrupt the Web and create a new (proprietary?) standard on how the new Web could be. They can, because one out of every six people that is online, has a Facebook account. They can because they have got such immense amount of data and people who are spending so much time on it, people will miss it when it is gone.
Facebook can, however will they do it?