Highly popular sites like YouTube, Reddit, Instagram, Quora, Pinterest, Dropbox, Bitbucket etc. are written mostly in Python. You would probably get a more comprehensive list by investigating on your own.
Obviously, the above list is not a complete one; but this would be a great time to point something out to you - writing your back-end in Python would not automatically make your site as fast and scalable as YouTube. We cite the list of sites above to simply point out that Python is a favorite of many software engineers in the industry, for web-development.
Python gives you the lego blocks to build robust, scalable web applications. You also get to enjoy the ease of writing Python modules while building it. But a weapon is only as potent as its master. If you don’t know how to wield it, you would end up probably shooting yourself in the foot.