Sending instant private messages in Facebook to a particular person is done through the following ways, after you are logged in to your account.
1. You visit the profile page of a person
2. Below their profile image, you get three options, Add Friend, follow, Message, More
3. If the individual is already in your friend list, even then you get the option to send a message in this way, which he/she will get to see when they open their Facebook Account.
4. If he/she is not in your friend list, even then you can send a message this way, although the messenger settings of that individual will decide, how your message will appear to them.
5. you can now install Facebook messenger as an android app on your smartphone, where you can send messages, video call, and make voice call to any profile you want.
6. But the voice calling and video functionalities will only be available, if that individual and you are connected to each other as friends.
7. When two people are not connected as friends, the messages will get filtered as other messages, and the recipient will only come to know about them, if they keep their settings that way.
8. Once they accept your friend request, all the above-said functionalities will get enabled.