Google is integrating Google Plus, its struggling bulletin board system (BBS), with other offerings such as Gmail.
One of the benefits is that you can post comments directly from your Gmail inbox, without having to using G+'s slow and clunky interface. One of the drawbacks is that Google can delete these comments from your inbox, without asking you.
G+ notifications look like emails in Gmail, and most users would consider it an outrage if their email supplier deleted emails behind their backs. So, if you want to keep these Gmail comments, you must either save them or forward them to another email service.
It's a good idea to do this anyway. Forwarding all Gmail to another service, such as Yahoo Mail or, provides a backup in case you are ever locked out of Gmail, or vice versa.
You might even consider switching away from Gmail entirely, on the grounds that you can no longer trust it. For example, someone who sends you an "email" message from G+ can edit it later, to change the contents, or delete it from your Gmail. They cannot do this if you use a different email service, beyond Google's control. (Tech experts may know these are not really emails, but I suspect most people will be confused by their appearance.)
You must obviously keep separate copies of your own comments, since G+ doesn't email you copies of these as part of the standard set-up. Indeed, if you are going to spend a significant amount of time making non-trivial comments, it's better to put these on your own blog, where you have control, rather than providing another service with free editorial to sell ads against.
One of the main topics of conversation on G+ is the awfulness of Facebook. However, when Facebook emails you other people's comments, at least you don't have to worry about them disappearing later.
Of course, comments on bulletin boards are always at the mercy of whoever started and/or owns the thread. If someone on a forum deletes their original post, all the comments usually go with it. When someone deletes a post on G+, the same thing happens. That shouldn't be surprising. What isn't normal is that email copies of comments get deleted as well.
If Google is going to do this, it should warn people in advance, rather than obliging them to learn the hard way.
Amusingly enough, I learnt this the hard way by making comments that were critical of G+ on a post from a Google employee/G+ evangelist. (It is, obviously, shocking that anyone should have the temerity to criticise G+ in public. Anyone who does this is a troll to be blocked, if not sent to jail without passing Go.) By deleting his G+ post and/or blocking me, Mr A Googler also removed the only visible copies of my comments. These could have been useful to anyone writing a blog post along the lines of "The top 5 reasons G+ deserves to die", though the system's failings may be too obvious for this to matter.
As it happens, I do have all my Gmail emails forwarded to another mailbox, but I don't think many people take this simple precaution. It could be important if there is a dispute.
Otherwise, I've just been distracted by some racy Facebook comments about journalistic shenanigans and PR failures. I wasn't wildly surprised when the group hosting these old stories seemed to vanish overnight, but I knew that whatever I'd missed would have been piped past my Gmail inbox into a Facebook "folder", so I could read them later. And I did.
Facebook 1 Google Plus 0.