Several MMORPGs have strong roots in books or RPGs – LOTRO (Lord of the Rings), SWTOR (Star Wars), Neverwinter Online (Forgotten Realms D&D setting) and the list goes on. I would say it’s likely that a good number of fans of the books / films / offline games or RPGs linked to these various imaginary worlds would have been attracted to the MMORPG by their connection to said world.
In the Internet age I would have thought that a good number of the fans of Lord of the Rings or Star Wars would know about the MMORPGs linked to those IPs if they care about gaming at all. For example if you search for “Lord of the Rings” in Google (.co.uk) the first link is a sponsored advert to LOTRO (same for Star Wars and SWTOR)!
But is it also likely that some people will have traveled the opposite journey, gone on from a big movie or book series to discover the pen and paper RPG hobby? RPGs seem to me to be even more niche nowadays than they were in the 1980s / 1990s; certainly the companies producing these games do not have the same marketing budgets that Warner Brothers, EA or other big game producers can bring to bear (such as paying for Google rankings).
As a purely anecdotal observation I’ve been looking up forums for D&D of late as I’m thinking of restarting a game I used to run and I’ve been somewhat dismayed at how (relatively) inactive the forums seem compared to the MMO forums that I frequent. I suspect that games like DDO, despite their popularity, do not necessarily help their ‘parent’ IP much. I doubt that the new Neverwinter Online MMO will do much for D&D either.
If this is true then it strikes me as a lost opportunity to introduce a wonderfully imaginative hobby to new audiences.