Near as I can find, the only on-line free backup service that supports both Linux, command line only headless mode  (like you'd find on a traditional linux server), and the ability to follow symlinks to arbitrary folders is Dropbox. The latter is important because you will want to back up an existing folder structure, and not have to put everything into a specific user's home folder

A good how-to is here:

One word of caution - the current release uses the binary file dropbox, not dropboxd, so you will have to edit the example init script accordingly. The whole process in general is a pretty fair hack, and not a great instiller of confidence, but it does work once set up.

Of course, all the other guys (such as Ubuntu One) are making these features available in the next version, so it may not be long until it's no longer a hack.