The abbreviation "BMS" stands for "broke my scale." This phrase is used in response to a "rating request" on Facebook: other users "like" the original poster's Facebook status, in exchange for a rating on their physical appearance. "BMS" is considered overwhelmingly positive: better than a perfect 10.
This is a Facebook phenomenon most popular with teens. A status update on Facebook might read: "Like for rate." For every user who likes the poster's status, she assigns a rating of one through 10 to their physical appearance. Similar statuses may read variations, such as "like for a secret," and the user shares a secret with every user who likes her status.