Lately, I've been working on behalf of some Forrester clients to answer the question, "How do we build a community?" Frequently, the answer is, "You don't build a community. You expand it." Few communities appear ex nihilo at the behest of a technology vendor. More commonly, successful community sites identify an existing collection of like-minded people, then entice them to your site with something of value to them.
Developer communities are a good example. Megavendors like Microsoft, Oracle, and IBM have an easy time creating a community site, since there are already a lot of developers connected to these companies, and to each other. If SAP never created its own community site, somewhere in Social Media Land there would be a collection of technical professionals talking to each other about customizing and implementing SAP applications.
Smaller vendors, of course, don't have nearly as easy a time. If you're a sales force automation (SFA) vendor, for example, there are plenty of people out there interested in sales force effectiveness, but far fewer interested in your tool. Since developers have no direct stake in managing a sales force, you can't count on them to be interested in you in the same way that a sales VP might be. Therefore, it's hard to see why developers would participate in a SFA vendor's forums, except when they have an immediate question that needs answering. (Usually in the form of, "Why doesn't this work?")