There are 6 search engines exclusively dedicated to search social network. Each search engine uses different algorithms to perform the act of searching.
The rate at which the social networking sites pop up each day, I am afraid they will be redundant in a year or two. Something different will be invented by then.
There is a new phrase that is being used in the social networking circle and it is called 'Social White Pages".
I read in Make Use of site that the search engine named SocialMention allows you to search blogs, microblogs, networks, bookmarks, comments, events, images, news, videos and audio.
Like what you do in Google, you can set up alerts for your searches and you will be notified in your email when an alert you have created found a match.