Facebook users often use the abbreviation term "PM" to describe both personal messages and private messages. Both forms of messages are methods by which Facebook users interact with one another. Each type of communication is slightly different, but the result is the transmission of a message that only the recipient can read.
Message
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If you wish to share a message with a friend on Facebook, you don't have to leave a posting on her wall. In the event that you don't want others to be able to read what you write, you can send her a private message. To do so, click on the "Messages" tab of your account, and then click "Send a New Message." Type the friend's name in the "To:" box and write your message in the box below.
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Chat
Another form of personal or private message is by communicating with a Facebook friend through the website's chat application. Click on the "Chat" icon on the bottom right side of the screen, and then click the name of a friend and type a message. If the friend has a green dot beside his name, he's currently online.
Security
Because Facebook allows you to fully customize your account's privacy settings, you may occasionally find a friend who has set up her account so that no one can contact her by private message. Additionally, some people may limit their availability to certain friends on the Chat application.
Use
Facebook users will often use the term "PM" when they're having a wall-to-wall conversation that they wish to make private. For example, if you've left a post on a friend's wall, and he's responded quickly, you may converse back and forth. If he doesn't want his friends reading the conversation, he may write something such as "Sending you a PM." In this case, you'll likely soon receive a notification at the top of your screen that you have a message in your Facebook inbox.