[brk]

I feel that I lose hold of my data

OK, I understand what you are driving at. I still am a little confused by the concept where you willingly post "your data" into the essentially open ether, then lament that action.

How about not posting it to Facebook in the first place? It seems that the implied contract with posting data into public places is that the data is in fact public.

What you propose is interesting, but I'm not sure it provides even basic security-by-obscurity. The keys would either have to be so overly distributed to your friends that they essentially become public. Or the data stream has the same message encrypted over and over again, which makes the size of the data grow exponentially (and also most likely makes it easier to crack).

[hypest]

I'm referring to the "friend-space", where info is public but only among these friends. I want to share stuff with my friends, and FB seems a rather good technology for that!

The public key is used for encryption, not decryption, so anybody can have it...To read a message I send you, you use your private key. The message will be encrypted by me using your public key. So, any friend can send me a message but only I can read it. If more friends are to be able to read the same message, the string must actually include an instance of the message, encrypted with the "other" friend's key.