There are a few ways to determine the length of a request, my favorite is a bug in chrome that lets you determine the size of a cached request. By abusing this you can tell if there are more or less results in a search result. As such you can go through the alphabet and determine the names in someones friend list on Facebook. Even if they keep their friends list completely private because you are having them do the search. See bellow for an example:

Keep repeating that until you find that they have a friend with the first name coldwater and then add a %20 and start again and you will find they have a friend with the name coldwater q

I use the mobile site because it has fewer side bars and such to make the content sizes more static