"Sometimes they'd see Ali's screen name online but when they tried to IM her, she wouldn't respond." Page:6 Wicked A Pretty Little Liars Novel

This is an important quote because some people would want to kill her if she wouldn't answer an IM. This is suspicious because one of the persons would have killed her but if her friends knew who the person was then they would have told the police who it was. They would be one step away of finding out who Alison's murder is. I think  Spencer, Aria, Hanna, and Emily won't find out who killed Alison. If they find out who killed there friend Alison then they would have to testify against the person.