So, I figured out that Emma was pretending to be Phoebe as soon as her name was mentioned on the revenge forum. Of course! Emma wants revenge, also Emma clearly feels guilty for something. This was pretty straightforward.
But, you guys, I thought the real criminal was Luis. I thought he had access to all the information, from the cafe and from getting along with everyone, and he was such a perfect boyfriend, I figured he had to be the evil one. He also had a little motivation, as the working-class kid with such rich classmates, and his perfectly friendly demeanor would be a perfect cover, too. I was so shocked when he was just a real nice guy.
Also, the ending? Where they pin the whole thing on the message board creep, letting little one off completely? Pure underhanded genius.