See those? They showed up on my site's stats under referrers after I posted my "ooh, shiny" icon, conveniently located on my own domain so I could see which pages it was viewed from.
That's fifteen sites (actually fourteen, there's one that's identical except for the lack of the www.) that grab images from the most recent posts. That's fifteen or more people who saw the image, and most likely had the option to visit the entry presented to them. Please be careful when you post images to LJ in public entries. My standard practice is to lock any entry that has a picture of myself, optionally unlocking it later.
I have seen a picture of one of my friends on one of these sites. (Taken from her own entry, thankfully)
You never know who's looking.
Also, these sites are fun to use, but don't do it from work. People post non-worksafe stuff all the time. :P
I found the webcomic "Sordid City Blues" (sordidcityblues) from its update being yanked by one of those sites. I also found some wonderful LotR fanart at mucun - Go visit her, that's one talented lady.
I'm not sure where I'd rate this on the creepy/nifty scale.. It could be used to meet people, which is cool.. though, the one time someone commented on one of my entries from seeing my picture of one of those sites, I was somewhat creeped out.
But what creeps me out are the people who look but don't let their presence be known.