MM a.k.a Mention Monday is the new craze on twitter. This will not replace but add to FF a.k.a follow friday. This is to let your fellow tweeps about awesome people to follow. Can be for those who may have missed out on Follow Friday. Also used for people you may have met over the weekend.
Tom: Hey man
Rob: WTF!? why didn't you FF me?
Tom: dude, dont worry ill just MM you
Rob: Oi, have you heard of Mention Monday
Tom: Nah, What is it?
Rob: this awesome new thing thats like Follow Friday but better!!!