I've updated the essential guide to include the MTU Facebook group, even though it's really not that essential.
FB groups also exist for TPL, Thai D1, Thai Port, Rajpracha, Bangkok Glass, Chonburi, BEC Tero, PEA Buriram, Chula United, Bangkok United (they even have an ex-pats section!) , Police United, Sisaket, Pattaya United , Chiang Rai United, KhonKaen FC, Chanthaburi FC , Bangkok FC (just one member! I joined out of sympathy) , Ubon United, Korat FC and probably many others.
If your team isn't there, why not start one?