WHEN ALL THE GIRLS HAVE GONE by Jayne Ann Krentz

When Charlotte can't find her sister, she becomes a little concerned.  But not too much because she has disappeared before.  When Charlotte tries to ask her sister's, Jocelyn, best friend where Jocelyn might be she finds Jocelyn's friend dead.  Charlotte finds two men at the apartment and decides to confide in one of them, a former profiler.  When other friends start to disappear, the story gets even more interesting.