Friendship forms an alternate network of alliance to family in the novel (it's like Facebook and Myspace – there's some overlap, but they're separate systems). The main difference between the two is that you can't choose your family, but you can choose your friends. This element of choice makes friends just as important as family, if not more so, in figuring out who a person is: the kinds of people a character likes reveals something about his or her values. Friends step up when family fails in the novel, providing another contrast between individual merit and blood ties.
By making Anne's friendships with Mrs. Smith and Captain Harville necessary to her ultimate reunion with Wentworth, the novel suggests that segregating friendship by class closes off beneficial opportunities.
The failure of Elizabeth's friendship with Mrs. Clay and Mr. Smith's friendship with Mr. Elliot suggests that equality is necessary to a successful friendship.