Uglies Series: Who Activated the Necklace? (Spoiler)

In the Uglies series by Scott Westerfeld, Tally Youngblood does not activate the necklace. The necklace is activated by Shay, Tally's best friend, after Tally gives it to her. Shay activates the necklace in order to find Tally after Tally is forced to turn herself in to Special Circumstances.