We have a lot of fans. By most metrics we have at least in the top three most fans in the world. There are also a lot of racist people in the world, and perhaps more racist people than we would have thought were still kicking around. Just by looking at those two things you would have to guess that we do have a lot of racist fans bouncing around. Hell, we have a lot of racist fans on here who end up outing themselves once they're in the right thread and are given the opportunity to do so.
Yes, it goes without saying that a club with a huge world wide fan base will have its share of (genuinely) racist fans.
However, this phenomenon (posting abusive shit on social media as an apparent reaction to players missing penalties) seems to be - well - real.
IIRC it was established that most of the users who posted abuse after England's Euro shootout were not based in England and most likely not England fans at all. *
It really isn't surprising that trolls (kids, like as not) have jumped on this despicable trend - just look at the response it generates.
* I didn't remember correctly: this was
rumoured to be the case but later proved incorrect. What
was established was that
a lot of abusive posts were made by people who very likely weren't England fans - but the
majority of the abusive posts came from users within the UK, so I was wrong about that.