From what I have seen STEMSocial is focused more on academic and professional level of STEM where STEMGeeks aims to be more casual and hobbyist friendly. Just casual discussions about new technology or science is happily welcomed in STEMGeeks where I always felt STEMSocial is more formal. Welcome that is more than welcomed in STEMGeeks I don’t believe the opposite is true.
I am always available if you had something in mind.