When you consult the profile of a member, the icon "message" allows you to send him a message. 

If the person you are visiting is connected live on the site, a blue "chat" icon will allow you to send them an instant message.

If this icon is grey, you will be informed that you cannot contact them because of filters in operation or because the person has ignored you.

On the mobile version the chat icon will turn pink if the person is currently online (the green dot next to the username also lets you know that a member is online on the site).

Web version : 

Mobile version :