I have been a member of Flickr for a little while now and find it is really good for motivation. The group participation and comments/awards that I get really keep me going, which is good because I'm terrible for not getting things done!
One thing that I have noticed on there, apart from the apparent popularity and appreciation of terribly overdone HDR images, is that some images get a huge number of comments and favourites in a relatively short space of time. One that I have just seen has been posted about six weeks and it has nearly 2,000 comments and has been favourited just over 700 times. Some I have seen with hundreds of comments and dozens of favourites that have only been up a few days.
There doesn't seem to be anything obvious to suggest how or why. Group contributions appear to be nothing out of the ordinary and the image itself is sometimes not what I would call outstanding.
I'm really curious to know how some people get so much exposure for their photographs on there.