Fame is not binary. Fame is not even analogue, strictly speaking, as in "on average, X% of people know who Jim is". Fame can be very dependent upon subculture. Among software people, you will find names like Linus Torvalds, Guido Van Rossum, Bruce Schneier, Jakob Nielsen and Scott Gunthrie quite well-known, but those same people are not famous in most other circles. Similar lists could easily be constructed for chefs, politics, journalism, music and any number of other fields. Fame is cultural, and culture is time and space and social contacts as well as shared experience, knowledge, beliefs and values. Culture is difficult to delineate, but it is the arena in which the amount of fame of one person takes on meaning.
Mokalus of Borg
PS - That's kind of circular.
PPS - I'm sure someone has done a better job defining these terms than me.