I find it effective because we all wander the halls of high school knowing about the girl who did that over the weekend, or in Melinda's case, over the summer. However, it is rare that we actually are the kid that did the act that everybody knows about. This puts us in her position, we see how she responds to the world through her eyes, the eyes of the victim of high school gossip and ridicule. We feel it. This is all so ironic because it is easy to be the gossipers and ridiculers and we are forced to live her experience as audience members.
I would say that the use of the first person helps to bring articulation to the idea of voice reclamation. Melinda spends the majority of the novel attempting to reclaim her voice. The world treats her with apathy or scorn, refusing to hear her speak. The use of the first person narrative allows this voice to resonate. While the world cannot understand nor seeks to do so, we as the reader, do. We begin to understand how perceptive she is and how insightful she is. The use of the first person helps us, as the reader, grasp how intricate and complex the adolescent voice truly is and empowers us to ensure that all voices can "speak," similar to Melinda's.
The first person narration is so effective in this novel because it allows us to walk in Melinda's shoes and hear her inner thoughts. The great irony in all of this is just that--for the majority of the novel, Melinda is unable and unwilling to speak; we, however, have the luxury of hearing her story.
Without the first person narration, we wouldn't be able to feel as strong of a connection to Melinda's plight and the emotional turmoil she feels in adjusting to high school and her new reality as a survivor of sexual abuse. Hearing the story in her own words helps make it "real" for the reader. We cannot avoid the pain and torment she feels and begin to experience it vicariously through her.