Promoting sub-communities of a social website?

I'm currently working on a social website that bases itself on specific sub-communities (something along the lines of Reddit meets Pintrest for lack of a better description). What are some effective ways to promote users involved in a particular subject to visit (and contribute to) the community without making it clear that the intent is to have them explore the rest of the site as well?

Look at how StackOverflow and how it approaches things.

There are explicit things you can do where you have a community that says things like "that would fit better over on site X." Alternatively, you can do things where you show "related" items from the sub-communities.

Realistically though, there's a major risk.. if you have one community with 100 people, it's better than 10 communities with 10 people. If you segment the audiences too soon, you could effectively make them wither to nothing. StackOverflow mitigates this by launching sites into beta and getting commitment from individuals before they formally launch.