If the Miss Bertrams enjoyed greater society than Fanny before they were out I agree it would be due in great part to Lady Bertram’s indolence and selfishness. Mrs. Norris actively tries to eliminate or curtail any pleasure that comes Fanny’s way and I feel the rest of the family are unthinking and neglectful. Edmund said of the trip to Sotherton that “She [Fanny] has not often a gratification of the kind” (8) which suggests she has had gratifications of the kind in the past just not often. I feel Mary’s asking if Fanny is out brought it to Edmund’s attention but a better spur to action may have been that Fanny expressed a desire to visit Sotherton:

“I should like to see Sotherton before it is cut down, to see the place as it is now, in its old state; but I do not suppose I shall.”
“Have you never been there? No, you never can; and, unluckily, it is out of distance for a ride. I wish we could contrive it.” (6)

If Fanny had continued a sparse social life as she aged it would have limited her chances of attracting a husband but I am not convinced a small society of family and “friends” before her ball was detrimental to her prospects. Maria was not acquainted with Sotherton or Mr. Rushworth before her second season of ‘winter engagements” (4). Julia did gain an admirer in Mr. Yates (20) just by staying home as did Fanny (30) and an unwanted one as well. Fanny has a pre-engaged heart so even had she more society it would have been unlikely to stir-up a husband—at least from her point of view.

Had the Crawfords not come to MP William would still have expressed a desire to see Fanny dance and inspire Sir Thomas to give them a ball. If Fanny was destined to follow the social path of her cousins she would not have come out until the winter engagements began which is around the time Sir Thomas gave the ball. I feel Fanny deserved to have more society than she was allowed but since she was not out just how much would she have mingled with unfamiliar gentlemen? (:D) Truthfully I do not see how Fanny’s prospects have been harmed by her lack of society. Fanny was admired at her ball and her prospects seem as good as they could be: