This simple tips will help you to easily identify your Facebook UserID without the need to do extra works (Such as right clicking your profile picture and Identifying your Facebook UserID yourself).
All you need to do is simply visit this page (change YOURUSERNAME with ..err.. your username, or if you want to identify someone else UserID you can just type in his/her username):
Your Facebook UserID along with other information such as your First Name and Last Name will be displayed.
Here’s an example:
Note: You can also use this method to get the ID of a Facebook Page.