Justin Theroux joined Instagram this week and is already proving that he knows exactly what he's doing when it comes to social media. The sexy actor posted a throwback photo of himself as a teenager on Dec. 3, captioned, "#TBT SIXTEEN." In the snapshot, Justin is seen rocking his signature earrings and long sideburns while wearing a dark jacket and staring off into the distance. Jennifer Aniston's husband also posted a black-and-white selfie earlier this week as his first Instagram photo. See the throwback below, and then check out all the celebrities you should be following on Instagram.