Can anyone share a link to a site showing the break down of market share among sites providing local search results (Google (Maps), Bing (Maps), Yelp, YellowPages.com, etc)?

I see some value in setting up accounts/listings in many of the low traffic sites since it doesn't take much time. However, I don't see value in checking search rankings or trying to improve search rankings in sites that might have <5% market share. Right now I'm on the fence as to whether to put any effort into Bing Maps/Local and Yelp beyond setting up listings because I don't know what % of local searches they get. What do you think?