We include the link to our social community in new hire materials. Also, many of our organizations have existing spaces, so they generally direct new hires to check out the spaces as part of their onboarding (and particularly if their systems or cubicles are not ready).
I guess, on a higher level, I'd say make the experience and the content so relevant that they HAVE to go there. Maybe work with HR to develop a page on the community with links to all the forms that a new hire has to complete. Create a simple five things to do list (complete a profile, introduce yourself etc.).
Hope this generates other ideas!