Evaluating Web Platform Systems

So you're shopping around for vendors, software and services to replace your current Web platform. Several questions no doubt go through your mind.

What are the features and functions we need? How can we use social media? Should we investigate SaaS or acquire a license and host our own solution? Or should we use a service to host?

What will the cost be? What are the due diligence steps we need to take with the vendor to make a sound decision?