This is pretty easy. Look at the location in your address bar when you’re on your Facebook profile page: the number after “id=” is your profile (user) ID.
That wouldn’t by itself rate a blog post, but I was trying yesterday to find my profile ID and was having some difficulty because I have a username and the profile ID no longer shows.
Google found me a number of “solutions” that involved going to your photo albums, picking out a photo, and parsing the address of the photo. What a pain!
Luckily there’s an easier way: