I also hate being blatantly advertised to... especially when it's under the guise of doing me a favour. Something like, lucky you, there's an update of x. Why don't you check it out and tell me what you think?
Somebody once sent me a PM that was basically a command. Check out my comic. Tell me what you think.
I think I'd prefer some honesty. Something like "Hey, I've seen some of your stuff and it's related to what I'm doing. I'm having some trouble getting off the ground and would really appreciate your opinion" and here comes the important bit, "If you could do this for me, I'd be happy to promote your comic on my blah blah blah page."
And if that person doesn't a) have any interest in the person they're messaging's comics and / or b) isn't prepared to make it worth their while, then they should really re-think the message.
I think these people must get super desperate or a bit cocky. They just aren't seeing the return they want or they think people will love their comic immediately so long as they can get people to look them. Some of the messages I've seen seem like copies of messages from much more established comics to their followers. But that's the important difference to me... If somebody has already subscribed to your comic, then they won't find it so weird to get a message telling them to check something out... The tone is more like a recommendation.
And I'm also grr at the lack of etiquette.
It's very soothing to think that these people are almost certainly eliminating themselves from the competition by using this technique. When somebody basically commands me, or spams me with notifications, I'm a stubborn mule and I punish them by actively not looking at their content. I'm guessing other comic makers are also not impressed by being told via a copy paste message to do something.