Just the other day I was having coffee with some cool people and we were talking about the future of search. I am a big believer that sites like those above, that are subsets of a larger domain, help return better results because they automatically screen the listings into a major category before you begin. Google has taken a similar approach with things like Google Scholar. These sites are nice because, like in the case of Flickr, you may search for something and get a ton of results. It's hard to narrow the results down to what you want, but sites that are more focused to being with may help.
The question is, can a group of sites such as those above combine and beat out the original Flickr? It depends. If the group of sites covers the right categories, mainly, the top 10 or 12 broad things people search for on Flickr, it might work.
Jennifer Rice once wrote somewhere that businesses should "pick a niche and own it." Great advice, but you have to pick the right size niche. Is Flickr's niche too big?