The only one site I can really recommend is Facebook, which seems pretty generic but most everyone uses it. Most cons will have a group if not multiple you can join for planning photoshoots, look for things like "SDCC Official," "SDCC Photoshoot," etc. My local con Sakuracon isn't anywhere near SDCC numbers but I'm a member of at least 3 separate FB groups for it.
You could also try Cosplay Amino, but I can't really speak for the site, I just know some people (I think mostly younger?) have been using it.
KuroNekoCon- '11 : Radcon-'12 : Akicon-'13 : Sakuracon-'13-'18 : PAX West-'11-'17 : Rainfurrest-'15 : ChibiChibiCon-'17-'18 : RenCon '17
Table Tennis Mei (Overwatch)