In case of Facebook I can imagine encrypting the data with OpenPGP keys into armored (text) format. Then you can place encrypted block to facebook or anywhere else. Other users would take the block, decrypt it on the client side and see it.
The same applies with other social networks and places where you can store some text block.
You can easily do encryption in some client application and even in Javascript (if you manage to make JavaScript load local user's keys somehow).