The project is still in the testing stage, and it's a bit finicky about how you enter character names. Make sure you're always hitting the submit button and not using your enter key, as otherwise the widget will keep searching for the last name you looked for instead of your new query. Its creator, Emilis, also wrote to warn that it uses live information from both armories and will occasionally be slow as a result. I imagine it might also be inaccurate if either Armory is having problems.
The widget is tremendously fun to play with and has yielded some rather interesting results even with the completely random names I keep trying. "John" and "Mary," as you might expect, are overwhelmingly Human toons, whereas the greater share of people playing a "Sergei" and "Yekaterina" are Draenei. 3 people with a "Brutus" are actually playing female characters, and 1 person with a "Laura" is playing a male character (Emilis notes that gender-bending names are surprisingly common, although from what I can tell so far this seems to be a lot more true of male names for female toons than the other way around). Most people with a "Killer" are playing a Hunter, Rogue, or Warrior. Characters named "Bank" are mostly Human Warriors, but "Banktoon(s)" are mostly Orcs. And, yes, most of the people playing a toon named Legolas are Night Elf Hunters. Are you really that surprised?
Thanks to Emilis for writing in!