Depending on who you are looking to book you, I would just mention that many large organisations block Myspace and Facebook, so that their employees can't access them. When I was working for the local authority, I used to get a myriad of speculative emails from artists, seeking bookings at events and providing links to their Myspace pages. I had to tell them that I couldn't access it and that if they wanted me to listen to their work, they'd need to provide a demo or a website with samples. So, there are still situations where an investment in a demo will be needed.

(The local authority concerned now blocks access to all music sites as well, classing it as Entertainment and making some people's jobs impossible, but that's another story - and no longer my problem!)