If you want to make your journal public, you certainly can, but by default I believe they are private. In order to 'add friends' to your friends list, your journal needs to be public, but you have control over which additional charts/graphs are shared.
To check your settings, just go to the PROFILE tab. I believe it's the last option on that page which allows you to make your journal public. Once you change the setting, it will provide you with the URL to your public journal and you can share that in your signature if you like, as I've done in mine below.
Hope that answers your question!