First, read Chris Brogan's blog and newsletter - he talks about these issues quite a bit, from the perspective of social media.
Second, you should always have an open line of communication with your users. Be aware that they are talking about you all the time, so try to make it on your turf. Set up a blog or forum on your site, start listening to what they're saying, and post answers to common questions there. Give your users a place to voice their concerns, and make sure that you have someone listening and responding. You should always be prepared to respond to their voice, even if it wasn't directed to you. (Are they talking about you on Twitter? What are they saying?)
Also, create a common place for making announcements and broad communications to your users. If you've set up a blog on your site (which I definitely recommend), then you can use that. Or just a note on the homepage of your site. Or call your users directly using the phone. Pick a method, and then stick to it consistently, and let your users know how you'll be communicating with them.
The frequency? As often as needed.
I think that Elie's advice is good for a class of customers such as mass market products but that does not work for everyone. If you're selling services to large company CEO's for example they are not going to your blog or forum site to get information.
You need to look at the specific people you want to reach and understand their communication style which will even vary within your customer base (your huge fans want to be communicated to differently than your casual users) and then create a communication strategy that works for each class of user.
You also need to realize that while social media is all the rage it not nearly as prevalent in many user communities as simple email. I've tried a lot of ways to reach our customer base (all electrical engineers) and when polling them the way they say they prefer is email.
The only effective way I've found to solve this is to start calling customer and asking them directly. It sure beats guessing.