If you joined CS50’s Facebook Group but don’t want to see new posts in your newsfeed, hover over any of CS50’s posts in your newsfeed until appears in the post’s top-right corner, then select Unfollow CS50 from that menu, per the below. You’ll still be in the group, so you can still visit it at https://www.facebook.com/groups/cs50 anytime you’d like. Note that old posts won’t disappear from your newsfeed (unless you remove them manually), but new posts will no longer appear. CS50 will thus gradually disappear from your newsfeed as other updates appear.