In addition to their fanfic, wasn't Clare also a journalist in her rl job?

There are plenty of other success stories moving from non-commercial fiction to commercial writing. A number of authors got their starts on AmberMUSH and a couple of the old Pern MUSHes. Jim Butcher, C E Murphy, Cam Banks, Angela Beegle, Rob Donoghue, Fred Hicks among others (note that some of those are RPG authors not novel authors at this point) but again they didn't capitalize on their fans from there. They used it as writing practice then went through the normal publishing process.