I happen to do this all the time without the hashtag. I will be adding the hashtag going forward.
Here are a few recent knitting selfies I have posted to Instagram and Twitter:
Wearing my "Wisp" cowl from Knitty Magazine (Yarn by MythicYarn)
Wearing my 'Striped Study Shawl' by Veera Valmaki (Yarn is KnitPicks Stroll Tonal in Springtime and Kindling)
Wearing a bulky, Alpaca, dropped stitch cowl (one of the designs that will be featured in an upcoming book - Yay)
I also post a lot of handmade sock selfies. My Instagram feed is basically yarn and knitting porn and my Twitter Feed is mostly the random thoughts that pop into my head with a spattering of knitting and yarn porn.
I hope I see more fibre enthusiasts using the #WearYourKnitting Hashtag.