This drabble takes place immediately after After Midnight, and reading that will put this into better context (though it isn't necessary).
"You're friends with John on Facebook," Sherlock asked in the guise of a statement the following morning over tea, as if ten hours hadn't passed since they'd moved on from that topic of conversation.
"Oh I've been friends with John for years," she said dismissively, flipping through a recent back-issue of The Economist. "Since before you and I met."
Sherlock looked up from his newspaper, and his demanding stare drilled into Irene's forehead until she glanced up as well.
"It was one of the ways in which I did my due diligence on you, and it's been a good way to keep tabs beyond the blog."
"But—"
"I used pictures of my very photogenic driver and invented an entire identity, 'Margo Zelle,'" she said, anticipating his question.
She closed the magazine and her lips curled into a devious smile. "It was quite fun, and just as easy as you might expect to reel in the dear doctor. He's rather forward you know, but Margo has been able to remain just elusive enough to keep him interested. I still chat with him from time to time to ensure I survive any social network cull, although now that he's engaged I might have to adjust my strategy a bit..."
"You flirted with John Watson," Sherlock said, his displeasure at this news evident.
For some reason Irene chuckled with delight at that, and she leaned towards him and asked with a provocative spark in her eye, "Jealous?"
Then without waiting for him to respond, apart from the withering look he threw her way, she corrected, "Not me, Margo," and went back to her reading.
Sherlock frowned, disgruntled with the idea of Irene and John carrying on a years-long flirtation, and worse, Sherlock being unaware of it. That Irene had just been using John to keep track of Sherlock, and that John was oblivious to her real identity were immaterial; he still felt irrational flares of irritation at his friend and possessiveness over The Woman.
In fact, Sherlock was tempted to inform John that Irene was still alive and well, just so that he could let John know that the woman he had been (unsuccessfully, he couldn't help but add) attempting to pull for so many years was The Woman, whom he'd believed dead.
It would defer some of the responsibility of the deception onto John, since he'd had plenty of opportunity to figure out the truth, but never had. Even better, Sherlock could also use John's inevitable guilt to manipulate him into being less angry over the matter, since John had always insinuated that he knew Sherlock had feelings for Irene.
Sherlock's lips pulled back into a one-sided smirk in anticipation of John's reaction. Yes, unless a compelling case came up, that might be just the thing to do for weekend plans.
But first...
The smirk dropped from his mouth, and with narrowed eyes he whipped open his laptop, logged into his own alias on Facebook, clicked in the search bar, and typed in: Margo Zelle.
Note 1: Sherlock is such a dick.
Note 2: The (in)famous WWI courtesan Mata Hari's real name was Margarete Zelle.