Social networks today large number. But the principle of action in all the same. Take, for example, "Vkontakte". The same condition for similar resources – registration. Go to the main page, click "Register" and receive your account. Fill in the fields with the information you want to share. Use the menu "Search" to find the familiar person.
When you find desired user, notice on his page. If under the picture it says "Send message", then the owner of the account admits the possibility of correspondence with unknown persons. Click on this link, type text message, hit "Send". If necessary, you can add a photo, document or music composition. Below links to the text resource is the inscription "add to friends". If you don't click it, you will have a limit on the number of sent messages, namely, twenty. When the user added friends, the limit correspondence is removed.
If under the photo there is no inscription "Send a message" means to be in correspondence with the user only after adding it to friends. Also you will not be able to write to his friend, while he in the black list. To start the communication, you can only after it will be removed from this list.