In the show Pretty Little Liars who is sending the text messages?

Mona, because she wanted to get revenge on the girls for how poorly they treated her, Toby because he wanted to protect Spencer, Spencer because she didn't want to be a victim of 'A' anymore, Ezra because he was writing a book about Ali, and Cece/ Charlotte/ Charles because she was pissed at the girls for being glad that Ali was gone.