Comment Exchanges in Facebook work like this, if you are a participating blogger then you must post your link to your blog entry in the official Comment Exchange post which is oftentimes started by an Admin of the group. Other participating bloggers will also post their own links to their own blog entries.
You must then visit all the links posted and comment on the blog entry. In return, other participating bloggers will do the same with your link. They will visit your link and comment on your blog entry as well.
Comment Exchanges are very common in the blogging world but I've never really participated in them until just recently. It's a very good idea of getting traffic and comments but it also has its own problems.
One of its problems is, some Comment Exchange participants don't even read what you've posted. They'll just base their comments on the title of your post and you will eventually end up with an out-of-topic or an off-topic comment.
For example, my blog post titled "Sizzling Bangus and Rice, Perfect Lunch For The Masses" received 19 comments but 7 of them had nothing to do with the topic. Those who posted the questionable comments simply read the title and assumed I was talking about a real Sizzling Bangus but if they had read the post then they would had known that I was talking about a brand of cheap fish crackers sold in Sari-sari stores.
Fortunately, the majority of those who commented read the entire post and was able to comment accordingly.
So, are you participating in any kind of Comment Exchange? If you are then do you think they are reading your post before they comment on it? If yes then post your answer as a comment below. If no then post your answer as a comment below as well.