If you have a Basic (free) subscription, you can only communicate with a specific group of users - specifically those who have purchased a Gold or Platinum subscription. These members have a C+ symbol displayed to the left of their username on their profile.
Essentially, a member who chooses one of these plans pays a bit more so that anyone can write to them, including Basic (non-paying) members. This option is not included in the standard Silver package; while Silver allows you to message and reply to everyone, you can only receive replies from other paying members.
As a Basic member, you can still complete your profile, perform searches, view profiles, and see who has visited you. Additionally, you can communicate with Gold and Platinum members - users who have paid for the ability to talk to everyone, regardless of their subscription type.
You can find full details on all our plans by visiting the subscription overview page.