Not strictly ebook, but SFF World has both Fantasy & SF book clubs...
http://www.sffworld.com/forums/forumdisplay.php?f=18
http://www.sffworld.com/forums/forumdisplay.php?f=20
...i09 has a SF book club...
http://io9.com/tag/io9bookclub/
...author Kelley Armstrong's forum has a paranormal/urban fantasy book club...
http://www.kelleyarmstrong.com/cgi-b...gi?act=SC;c=11
I'm sure there are Mystery, Romance, etc. clubs on other forums too.