The Only Girl in Town by Ally Condie

 
For July Fielding, nothing has been the same since that summer before senior year.
 
Once, she had Alex, her loyal best friend, the one who always had her back. She had Sydney, who pushed her during every cross-country run, and who sometimes seemed to know July better than she knew herself. And she had Sam. Sam, who told her she was everything and left her breathless with his touch.
 
Now, July is alone. Every single person in her small town of Lithia has disappeared. No family. No Alex or Sydney. No Sam. July's only chance at unravelling the mystery of their disappearance is a series of objects, each a reminder of the people she loved most. And a mysterious message: GET TH3M BACK.