Contracts are the usual form of defining an agreement, they are an enforceable substitute for trust that each party will keep his end of the bargain.
However, the web has made information so freely available, that the potential is for a substitute form to evolve, a form that calls for, and publishes performance data for all to see.
The ambient threat that non performance to an agreement will become public knowledge is going to become as powerful as a contract, as it will inform others that your business is not to be trusted. Similarly, the converse is true.
Will facebook replace a contact, perhaps not, but it may mitigate the current monopoly lawyers and the courts have on the process of agreement enforcement, becoming entirely more democratic and affordable in the process.