"Both of them used Twitter to send vague messages to each other, but one of the last straws was when a photo of Justin surrounded by Victoria's Secret models was posted."

According to the pal, what started as a break between Justin and Selena turned into a full breakup when Selena discovered a mutual friend posted a photo of Justin with the models from the night before.

Justin Bieber and Selena Gomez's breakup shocked fans, but pals close to the one-time couple saw it coming and exclusively tell RadarOnline.com that exhaustion, jealousy and a busy work schedule are to blame.

"Selena was pissed and immediately texted Justin, which caused the friend to delete the tweet," the insider revealed.

"But not before Selena retweeted the photo of Justin and the Victoria's Secret model with the caption '…' which she later deleted."

As RadarOnline.com first reported, Selena was trying to stick it out with Justin because "he's good for her brand," but now Justin's reputation is slowly changing and Selena is fine as a Disney sweetheart on her own.

"When they originally started dating, Justin was a boy entertaining young girls, and now he's a man surrounded by Victoria's Secret models…and Selena doesn't like that," the source said.

"I can tell you that right now the biggest debate over at Team Selena is whether or not she should unfollow Justin on Twitter," the source said.

Just one day after news broke that he and Selena Gomez were no more, Justin Bieber chose to perform the most famous break up song in recent pop music history ... Justin Timberlake's "Cry Me a River."