...between her and Mrs. Gardiner, with occasional help from Elizabeth, the conversation was carried on. Miss Darcy looked as if she wished for courage enough to join in it; and sometimes did venture a short sentence when there was least danger of its being heard. (Ch. 45)

It's interesting that it was the women of less standing (a tradesman's wife and a paid companion) who were carrying the conversation.

I don't think that JA wanted to imply that only persons that were good at conversasion were well-bred, in that case that would mean that shy Georgiana was not well-bred. I'm sure that Caroline and Louisa were great conversasionalists when they wanted; they must have been conversing with Georgiana before Elizabeth and Mrs. Gardiner enter; they just didn't talk to country girls and their aunt from Cheapside. :)

Mrs. Reynolds says about Darcy:
"Some people call him proud; but I am sure I never saw anything of it. To my fancy, it is only because he does not rattle away like other young men." (Ch. 43)

We know that Darcy is proud and that he didn't like to talk to people in Meryton, but perhaps he was always the reserved type, even at his own house.

[H]e frequently sat there ten minutes together without opening his lips; and when he did speak, it seemed the effect of necessity rather than of choice -- a sacrifice to propriety, not a pleasure to himself. ... Colonel Fitzwilliam's occasionally laughing at his stupidity proved that he was generally different (Ch. 32)

IMO this not necessarily means that Darcy prattled in other places, just that he conversed a bit more.