According to the Official Press release, it is clear that Facebook will not be free for ever. Facebook has start charging 1$ to send message to any person who is not in our friend list, but currently this experimentation is limited to US only but soon will be charged in every country.
Facebook has taken this step to improve the security so as to minimize messages from unknown sources. This 1$ is also charged for the priority of the message. Currently this amount may change as the situation goes or may vary according to countries.