Hi Lucy,
Welcome to DotNetNuke! There are certainly a number of CMS options out there, so the choice can be difficult. I've worked with or considered Xoops, Mambo, which is now Joomla, Plone, Drupal, Rainbow Portal and recently Django and in the end I've settled on DotNetNuke because of two major considerations: the size of the community and the architectural design of the CMS infrastructure. One of the classes I used to teach as an MCT was Advanced ASP.NET development and the focus of this class was basically on the desing principles implemented in the IBuySpy portal which was the predecessor to DotNetNuke.
One of the really exciting things about DotNetNuke related to your business is the fact that the next major release, Cambrian, will focus on some of the social networking and social media features that you seem to need. There is also a vibrant community of developers out there who are working to build both free and highly affordable solutions that will work well for your business goals.
The one drawback I see for you is the learning curve with .NET if you plan to do a lot of the development yourself. On the positive side, DotNetNuke is written in Visual Basic.NET, which is one of the easiest languages to learn, and there are great resources available, both free and in the form of paid subscriptions, to help ensure that you have great support as you go about designing your solution. If you haven't already checked out some of the videos available from the resources section of DotNetNuke.com, this is a great place to start.
HTH and Hope to see you in the forums!
Don