They have 400 million users and 500 million people visit the site each month. Only Google, Microsoft and Yahoo have more monthly visitors than Facebook. And only Google has more page views. And they aren’t done growing yet. In a year they will likely be second on the list of unique visitors. In two years, they’ll probably be first.
In all the discussion about the open graph release by Facebook (more here), I think this is the real point behind the story. Facebook has done an amazing job of using the power of the network effect to build a huge, active and growing user-base. Even without these announcements the company will likely reach the kind of market penetration that can only be dreamed of by other web companies.
The fact that Facebook is using this position to change the web makes logical sense - they hardly need the lock-in.
If we're honest about it, there is only one "open" that would really count which would be that a facebook user could freely friend a a user of another platform and vice versa to set up the same relationship they can within Facebook.
Perhaps this will even happen in time - and facebook will turn itself into a utility service - however requiring this from Facebook is the same as asking Google to open source pagerank, all the data they use to serve ads etc. etc.
In the meantime I hope there'll be some kind of equilibrium of playing nice with web standards. Perhaps other players will figure out how to make us create / maintain Google, Microsoft, etc. identities instead of or rather then facebook identities.