While there has been a gradual decrease in every measure of gregariousness, not just teen pregnancy, since I was a kid myself, the marked impact of smartphones is astounding.
Have a read at this article. You'll be safe, it's from The Atlantic - no Breitbart quality shilling here!
Not sure what you mean by gregariousness. While I agree that smartphones are having serious impacts on our culture, I still dont think its safe to say that this is due to cellphone usage.
Interpersonal interaction, face time, people belly to belly. Gregarious people are those that interact socially more.
As TV rose, it very slowly degraded social intercourse. The rise of the internet made it worse.
Cell phones show it might be something that can just be prevented. AR is just coming on the scene, and VR is at it's heels.
It's difficult to even imagine people are going to interact more in person as our phones replace our friends and neighbors.
Have a look at the charts on the link I pasted. It's pretty obvious. If you read the article, it discusses how phones keep us apart, and how that impacts our kids.
It's not a positive influence.