a & b) StackOverflow itself is very specific and is probably a better example of having a specific market with pro users. c) ChaCha may have comments but, like

a & b) StackOverflow itself is very specific and is probably a better example of having a specific market with pro users. c) ChaCha may have comments but, like in (b), who participates in them? Knowledgeable members of the field? Definitely not for ChaCha.