Does this mean these companies are outsourcing their graphics, mock ups, etc.?
On the other hand "Used by X" can mean anything from "X uses us as a core part of their workflows" and "A small team at X uses this sometimes". So don't give it much weight.
For example, even though Salesforce obviously has a lot of talented programmers on the payroll, I wouldn't be at all surprised if the Dreamforce conference website is built by a separate firm.