Sherlock loves his iPhone. It has everything on it: important photos for cases, music, he even uses it for games.
Sometimes, when he gets bored, he likes to check the top app charts to find interesting new apps or games.
Which is how he found Instagram.
He started out slow. He followed the users the app itself suggested, he learned about commenting and liking, he found out what hashtags are, he even found some users to follow on his own.
And then he started posting.
The first photo was a blurry photo of a dead body. The only reason John let him leave it up was because it was blurry, and you couldn't really make out much of it unless you were a) trained to see such thing or b) present when Sherlock moved to snap the photo and John smacked his phone clean out of his hand.
The second picture was one of John napping on the sofa. (John repaid him later by downloading the app just so he could post a similar photo of Sherlock).
After the tenth photo was posted via Sherlock Holmes four weeks after he downloaded the app, fans from his (John's) site began to follow him. Somehow they found him, and Sherlock gained 100 followers in one day.
Sherlock liked the attention, of course. Since the followers weren't annoying paparazzi or physically in his way, he liked when his photos got "likes".
Once, he even posted a photo of John making breakfast (Sherlock was sitting at the table and John's back was to him), that got over 500 Likes. Additionally, it had almost an equal number of comments along the lines of "Aww!" or "How cute/sweet!" Even though Sherlock hadn't meant for the photo to be cute or sweet.
Another time, Sherlock posted a photo of John shaving. It got similar comments.
"You notice he posts a million pictures of you?" Lestrade asked John one evening while Sherlock was examining a body.
"Hmm? You follow him?"
"Me?" Lestrade snorts. "No. My daughter does. She finds the photos of you sweet."
John laughs. "I hadn't really noticed. Guess I'll pay more attention when he has his phone out."
For a while John didn't mind Sherlock posting photos of him. He liked the attention Sherlock gave him; after all, his website is about Sherlock. Sherlock's Instagram could be about him.
But then it began to get annoying, mostly after Sherlock posted a photo of John actually smiling and looking sexy-John-like and all the comments were, "Sherlock you're so lucky, John is so hot!" (The WORST comment was, "John is such an attractive older man!")
The final straw for John, though, was when Sherlock posted a photo of him in just his pants. Honestly, Sherlock didn't even notice. The focus point of the photo was the blood swatch Sherlock was holding up, but John was in the background, standing in the kitchen wearing only pants.
Forget just deleting Instagram. Sherlock's phone didn't make it through the night after all of the notifications came pouring in. John grabbed Sherlock's phone and smashed it with a hammer, and after he replaced it, Sherlock had to promise never to post photos from inside the flat ever again. And that was after he was banned for Instagram for six months (two months longer than he even had it).
*I have a headcanon that Sherlock is obsessed with things on his phone like Angry Birds and now Instagram. I mean, come on. For a man that gets that bored they could be entertaining.