Notes:

My edit using screenshots from Gravity Falls to describe various apps and websites. I didn't see this one until after I started making this, but I feel it's different enough to be okay.
Screenshots from the following episodes (and one short):
Facebook: "Headhunters"
Twitter: "The Love God"
YouTube: "Fixin' It with Soos: Golf Cart" short
Instagram: "Dipper and Mabel vs. the Future"
Snapchat: "Scary-oke"
Pinterest: "Headhunters"
Wikipedia: "Double Dipper"
Twitch: "The Hand That Rocks the Mabel"
OkCupid: "Tourist Trapped"
Myspace: "The Inconveniencing"
4chan: "Soos and the Real Girl"
Reddit: "Gideon Rises"
Tumblr: "Boyz Crazy"
FunnyJunk: "The Inconveniencing"
DeviantArt: "A Tale of Two Stans"
LiveLeak: "Sock Opera"
Steam: "Boss Mabel"
WorldStarHipHop: "Dipper vs. Manliness"
Pirate Bay: "The Hand That Rocks the Mabel"
Silk Road: "Not What He Seems"
9gag: "The Stanchurian Candidate"
Vine: "Weirdmageddon 2: Escape From Reality"
Fur Affinity: "Boss Mabel"
iFunny: "Soos and the Real Girl"
Deep Web: "The Last Mabelcorn"