This has been happening to me for years. When I was in high school back in 2004, someone lifted all of my fanart from my favorite video game forum and pretended to be me on deviant-art because I had no presence over there and they were sure they could get away with it.
I've had my watermarks erased countless times and my art modified for so many different things, icons, banners, reaction images for character-specific roleplayers. After 2011, I just stopped watermarking and said "Fine, go ahead, steal it, I don't care anymore." There's more of my artwork on deviant art today than there was the first time it was stolen.
I've seen people redraw my artwork, vector my artwork, make templates out of my artwork.
I've even had a best friend steal my original artwork and take credit for it on an anime forum. We haven't spoken since.
There are people who have sourced my fanart as official art. There are people who try to credit me and misspell my name, and people who credit me as a pixiv artist even though I don't have a pixiv account (unless someone is over there pretending to be me IDEK). Even though I have an art tumblr, my pictures still get reposted by other users. I've even had thoughtful users invade my inbox, demanding that I credit my own artwork because they were so sure it wasn't mine.
And of course Instagram, Pinterest, Twitter, and Facebook.
The biggest steal that happened this year was during the week of the Eurovision Song Contest. In the Hetalia fanbase, we have a tradition of blowing up our community with crossover fanart. My sister showed me, the moment it happened, that the contestant for Moldova found my fanart and posted it to her facebook to show her fans and dress designer. I was embarrassed as hell because it wasn't a picture of HER, it was a picture of HETALIA MOLDOVA wearing her dress, so I literally just walked away from it and tried to pretend it never happened, but a bunch of Hetalia fans jumped into the comments and explained it to her that it wasn't her in the picture and I was the artist. That was quite nice of them,but then the fandom proceeded to pat the fandom on the back for getting noticed by Eurovision-senpai and that part alone made it feel like the worst art theft I've ever experienced.