My mother-in-law announced the news of my pregnancy on her Facebook page, before I even told my closest friends. I am so angry with her. My husband says i should let it go. How should I handle this?

Actually this shows that your MIL is very excited and happy over your pregnancy, and she wishes to share this joy with her friends. You can try to think positively over her action instead of thinking badly on her. If you do not like her to do this again in future, you can politely tell her to seek your permission before posting anything on FB that is related to you.