Facebook lets friends connect. They can give each other updates, share photos and post comments. But that’s not all. Facebook might also stress users out. The evidence: elevated levels of a tell-tale hormone in the saliva of users with lots of friends on the site.

In a new study, researchers asked 12- to 17-year olds to complete questionnaires. The 88 volunteers reported how much time they spent on Facebook. They described how many people they had friended through the site. Each person also answered questions about the types of Facebook posts and comments that they made.

In addition, teens and tweens in the study answered questions about their feelings and state of mind. Some questions sought out signs of stress and depression. (People with depression generally feel sad and may lack energy for day-to-day activities.) Other questions asked for the Facebook user’s feelings of stress and self-esteem. And some questions asked how much support each person felt he or she got from other people.

The volunteers also gave saliva samples. This happened four times a day on two different days. Researchers tested those samples for cortisol. This chemical is a stress hormone. Levels of it vary throughout the day.