When I was first working through the Facebook API binding, there was some stuff changing within Facebook's Graph API for sending messages between users. It hadn't settled down before I pushed the 1.0 release and so 1.0 went out without that capability.
I've been monitoring the Graph API keeping watch for this to be available, but all indications are that there is still no way to send private messages to other users (and due to the risk of spam, there may never be). I just did a scan of the Graph API documentation and saw nothing that would allow for this. That said, it looks like the inbox and outbox endpoints are available for reading messages that a user has sent/received. This is an opportunity for an improvement to Spring Social Facebook's API binding and I've created https://jira.springsource.org/browse/SOCIALFB-85 to track this improvement.
Spring Social Project Lead