Facebook groups perhaps? I'm a moderator for a group Stage IV Prostate cancer...
I believe you are correct that FB has likely taken much of the traffic from message boards. Personally I hadn't used FB until my mesenteritis diagnosis last fall, and I could not find any online support groups other than a small presence on FB.
Also I'm not in any FB prostate groups, but started several private groups for specific conditions for which there did not appear to be any existing groups. Women seem to outnumber men there 4 or 5 to one. But I think men may feel more comfortable with the message board format as many of us were involved in the early days of home computing thirty years or so ago, when message boards flourished.
2014-15: PSA's 9, 12, 20, 25, Neg DRE's, false neg TRUS biopsy
6/16: MRI Fusion biopsy, 6 pos Rt Base, 2x40%+2x100% G8(4+4)
8/16: DaVinci RP, 3 foci EPE, PNI, 11 LN-, 53g, 25%, BL SVI, pT3b
1/17: 18 months Lupron, PSA's ~.03
5/17: AMS800 implant, revised 6/17
8/17: 39 tx RapidArc IMRT (70 Gy)
1/18, 4/18, 7/18, 1/19: PSA's <0.008, T=9
2018: Dx Radiation Colitis and some other weird stuff