Go to the profile of the person you want to send a message to, and click «Message». A chat window will open, which you can use to send messages. Once you have established a correspondence with this person, you'll be able to access your messages to them in the «Messages» section.
The message recipient will receive a notification that he/she has a new message in «Messages». Until the recipient reads your message, in the chat window beneath your message it will read «Your latest message has not been read yet». If this notice has disappeared, it means your latest message has been read by the recipient.
You'll receive notifications of new messages in the «Messages» tab, where you will see a picture of the person who sent you the message in the bottom right corner of the screen. If you enter this section, you'll see a notice next to the folder which has new messages.
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