Yes, unless you request otherwise, company reviews on Viadeo are strictly confidential and anonymous by default. In order to ensure that you remain anonymous, take care not to mention any information that may reveal your identity.
If you wish, you can link reviews to your Viadeo profile, which clearly removes all anonymity. In doing this, the company and other Viadeo users can identify you as the author of the review and get in touch with you.
Your reviews are extremely valuable to both the companies and the entire Viadeo community.
Help the community by reviewing companies you have worked for.