I recently upgraded to Ubuntu 13.04 and when setting up the online accounts for face book it said Ubuntu would like to post to your friends on your behalf. What does this message mean?
This means that when you post a message on facebook via some ubuntu app that uses online accounts, practically you are just submitting the message to the ubuntu app.
Its then the app, who posts your message to facebook, on your behalf!
In the permissions line that your question mentions, "the app" is abstracted to "Ubuntu", and the final line reads: "Ubuntu would like to post to your friends on your behalf".