There are many online cake sites where people post photos of cakes that they've made. Add flickr and facebook into that, and you have a tremendous number of pictures to look at or to comment on.
Having said that, who really has the time to comment on pictures? Personally, I hardly ever look at pictures of cakes online, unless I'm looking for something specific to show a client, or if a friend has posted pictures.
When I post a photo I really don't expect to get comments. They're nice if people feel like they want to take the time, but I'm not insulted if people don't. I look at posting photos of cakes as a type of advertising, not as a way to get compliments.
Plus, when I see some of the "awesome cake" comments that people have put on photos of cakes that were clearly made by a blindfolded monkey, well, the "nice cake" comments lose some of their luster.
One cake forum will periodically have someone post that nobody is making comments on their cakes, and they're sad because of it. I usually go to look at the photos then, being curious as I am. Many times, though, the reason why there are no comments is pretty clear.
If a cake looks like it was dropped off a building, and nobody comments on it, it's probably because nobody wants to say that it looks like crap. Count your blessings if nobody comments.
I was watching Kitchen Nightmares, and the owner of the failing restaurant was saying that people were always telling the waitstaff how good the food was. Gordon Ramsay responded that the servers in his restaurants were trained to listen to the bad comments, not the good ones "where people are blowing smoke up your ass."
If you're asking for comments, then comments about things that you can improve can be more helpful than smoke up the wazoo.
People generally don't want criticism, though, they want everyone to gush over them. It's obvious to me that there are enough people who are willing to post "that cake is amazing" on a cakewreck that you can usually get one or two gushy remarks on even a horrible cake.
It doesn't mean anything, though, if all you're doing is cutting out cartoon characters from a color printer and slapping it on a monkey-iced cake.
If you really want people to comment on your photos because you really want to get better, you should spend more time practicing and less time fishing for compliments. If you're just wanting to hear people tell you how great your pile of cake is you'll find that, but it means nothing. So figure out before you ask WHY you're asking for people to comment. Then be prepared for when they do.
Or maybe you should just go practice some more and not worry about it so much. That's a lot less stressful.