It's much more expensive to expand your phone network and network effect between distant physical locations almost does not exist.
How much does it cost facebook to handle traffic in India? How much would it cost for american-only phone networt to create and connect new network in India?
So if we handwavingly turn the argument around: if Facebook and others had to physically build their own networks, we would have had an open social network :)
But now that they can use the internet, the networks become closed.
How much does it cost facebook to handle traffic in India? How much would it cost for american-only phone networt to create and connect new network in India?