There are two methods of communication – synchronous & asynchronous
There are two categories of communication – broadcast & private
And there are response two timespans – immediate & anytime

You need to build a platform around one or more of these. Ideally all methods should be supported, but each platform is best for one of each

Email: private, anytime
Instant Message: private, immediate
Chat Room: broadcast, immediate
Twitter: broadcast, anytime
Phone: private, immediate
Blog: broadcast, anytime