Probably some people would kill me for that answer but i would say you should learn PHP.
PHP is simple, good documentation and great community.
Facebook is built on top of PHP so as Drupal and Wordpress.
But the main point about my recommendation is that PHP is installed everywhere, you don't have to spend much time configuring and doing sysadmin stuff to make it work.
I started with Qbasic and did my first more ambitious projects in PHP. I agree that it is really easy to get started, but the horrible security holes I created are the downside;-)
PHP is simple, good documentation and great community. Facebook is built on top of PHP so as Drupal and Wordpress.
But the main point about my recommendation is that PHP is installed everywhere, you don't have to spend much time configuring and doing sysadmin stuff to make it work.