Found it! I'd been wondering for a while what "MM" means in a Chinese language context. It was really obvious, I can't believe I didn't guess it right off. I knew it had something to do with cute girls, from seing it in contexts like these:

[MM热图]一组特别出色的NEC MM照片
Hot MM Pics - Outstanding Photo Collection of NEC's MMs(Source, see SFW photos)

帮帮兄弟出个主意，我真的不知道带MM去哪里谈情说爱才好。
Help a brother out; I really have no idea where to take an MM to profess my love.(Source)

But it wasn't until I was reading through a dating-advice thread on the MIT BBS that I realized, "mm" means 妹妹, literally "little sister". Correspondingly, "gg" is 哥哥/"big brother" and "dd" is 弟弟/"little brother":

看看他对你们周围的中国人是否也特热情de关心中国文， 特别是对ggdd, 已婚中国mm
Look and see if he's also interested in Chinese culture when he's with other Chinese people, especially Chinese GG, DD, and married MM.(Source)

(Based on the information she gives, I think she should dump him. If he has kids, not many other friends, and has to be so insistent on setting up a date (showing up at her house unnanounced!) then he doesn't deserve her. I think she will find better out there.)

The next mystery is to figure out what "么" means in sentences like 安全第一么。. Hmmm...