This article confuses some things but try it. I didn't get it at first either but it's a great idea and crazy easy to use. If you have tons of stuff spread out over multiple social networking sites, there's no easier way to search through your own online history. And our histories are just going to get bigger and bigger.
I can see the privacy concerns but you can select which platforms you want (or don't want) Greplin to have access to, so that's not such a big deal to me. For sites people use professionally, for organizing projects etc this is a great tool.