Most Helpful Guy
Because he doesn't want to talk to you anymore or deal with you on social media. Out of sight, out of mind. He's moving on, you should too--after all, YOU broke up with him.1
Most Helpful Girl
Because reality didn't sit in until you started adding other guys to your account. He thought that you weren't serious and had intentions on possibly giving you a ring or leaving a message but all of that changed.
I really don't know what his motive was before you guys started dating 11 months ago, but I could tell that he wasn't all of that serious about getting in to a committed relationship with you. You might have made him number one and respected him as someone you were interested in but he wasn't doing the same unto you.
Good luck with everything!1