Although Emma has, at times, a snobbish attitude by modern standards, I don't think she ever actually snubs anyone. A snub requires dropping/declining an acquaintance, or at least cold behavior (of the "I'm going to invite you to tea, be perfectly polite, and make you feel like the worst piece of scum that ever existed on the face of the Earth" kind). She may think that the Coles, the Coxes and the lot are vulgar etc, but she is always gracious to them (and finally goes to the Coles' dinner).

As for Miss Bates and Jane Fairfax, Emma is bored to hear the soliloquies of the one about the perfections of the other, but that's not quite the same thing as a snub. She could be more enthusiastic in her visits, but she does visit (and send provision from Hartfield; in that she is not neglectful) so we can't say she snubs them. She also keeps seeing Mr Elton after his marriage. Granted she thinks him beneath her, but, as far as I remember, she keeps these thoughts to herself (also, socially speaking, he is beneath her).

As for Robert Martin, first of all, we don't know that he wasn't introduced to her. We have no dialogue for this part of the story, and Harriet could well have said "Miss Woodhouse, may I present..." etc, and then Emma would have walked forward and let them talk. If this had not happened, it would still not count as a snub. When two persons met, the first step to be acquainted belonged to the socially superior, if (s)he was interested (as Elizabeth reminds to Mr Collins). If such a step is not taken, it's not a snub, simply lack of interest. A snub would be if Harriet asked "Miss Woodhouse, may I present to you Mr Robert Martin?" and Emma said, NO. And I can't see her doing that.

However, she did orchestrate Harriet's snub towards the Martins, and that was indefensible.