As part of the 'A Message From Earth' project, Bebo users can create their own images or text via an application and vie for a spot in the final 500.
The messages to be sent will be chosen via a web vote, which runs until 30 September 2008.
"A Message From Earth presents an opportunity for the digital natives of today, for whom the internet is both a fact of life and an integral part of their lives, to reconnect with science and the wider universe in a simple, fun and immersive way," said Mark Charkin, vice president of Sales at Bebo.
The winning messages will be broadcast on 9 October by the National Space Agency of Ukraine's giant RT-70 radar telescope and are expected to reach their target during spring 2029.
Oli Madgett, the man who conceived the project, reckons it will help "to use the angle of space to help capture young people's imaginations and inspire them to think about our own planet earth and humanity's impact upon it, as well as their personal worlds and the things that they feel should represent them."