Google's great at bringing people to your site, and Joomla's pretty well tailored to helping that happen (hint: use keyword and description META tagging, post regularily and turn SEF on!), but you can also do more to promote your site by empowering your community.

For a long time, mambo/Joomla's had the ability to let users email short messages to their friends specific to the content they're browsing - its stock with a base install and can be turned on/off from your general configuration settings in the back-end.  I recommend you leave it on.

Additionally, there are a few extensions you can use to create specific areas on your site to Spread the Word about your site in general.

Here are a couple of options: 

Online social networking sites - like friendster and myspace, now boast membership in the 10s of millions of people, from around the world.  This means that people are increasingly looking at the web as a place to interact, above and beyond simply reading text and looking at pictures.

I'm going to be quick and state something obvious to ardent fans of this CMS, and inspiring to those considering using Joomla:

Joomla lets you add communication functionality to it, as you see fit.

Now, when we're talking about enabling websites to be places for people to meet each other, communicate about common interests and so on, J! is particularily positioned to do this in a way that is contextual.