Prompted by a query from Sabine Hossenfelder, I've just changed the comment system on my blog. When it looked like Google+ (which is Google's mostly failed attempt to take on Facebook) was going to take off, it seemed quite sensible to take Google's offer of switching the comment system over to Google+
The good news was that it meant any comments on a blog on Google+, but I hadn't realised until I just looked into it that it also meant that you could only comment if you had a Google+ account.
As a result I've switched the commenting back to Blogger's own. But the downside of this is that some existing comments will disappear. Many apologies if this happens to one of yours. I promise not to change it again!