There's been plenty of conversation about Fifty Shades of Grey, the mega-selling book that had its origins in Twilight fanfic. Now, I'm not going to knock on Fifty Shades here. I haven't read it, and I probably won't, because it's not my kind of thing. However, I don't begrudge its existence or its success. It's really great as far as I'm concerned: in publishing, a rising tide raises all boats.
Because, here's the thing: on some level, we all start out writing fanfic. Now, it might not be a conscious choice of, "I'm going to write Star Trek, except place it on a sailing ship of a fantasy world", or, "I'm going to write Heinlein-esque space opera, but with less incesty group sex". But those influences can be really obvious. When I wrote the trunked Fifty Year War, I had been reading plenty of David Weber, and those fingerprints are ALL OVER the manuscript*.
However, sooner or later we get deft enough at filing off the serial numbers, or melding our influences into a distinct voice, and suddenly we have something fresh and unique. I think every writer has that moment. Most of us don't have our original source as well documented as Fifty Shades-- which used its fanfic origins as a marketing tactic-- but most of us know what they are.
Though that is something that should be noted about Fifty Shades: no one should be looking at it as a viable marketing strategy. It's lightning in a bottle. You can't plan on catching it.
*- As in, lots of meetings where groups of politicians and military advisers sit around a table and talk about what's happening. Yeah. FYW is trunked away for a reason.