OK, 2nd stupid question today I guess...but I don't see where you leave comments for apps that you've downloaded...must be something obvious I'm missing.
Go to Android Market > My Downloads > App you want to rate > Hit Rate It! and give it a star rating of 1-5. It will then give you a link whereby you can write your own capsule review of the app.
Unable to update play services update
Facebook
Twitter
Instagram