The new Greplin service is like desktop search, only it indexes online accounts. It uses OAuth to get personal posts and timelines from Twitter, Facebook, Gmail, Google Docs & Calendar, Dropbox, and LinkedIn*. The Greplin service is cloud-hosted, and the company says that it updates the indexes around every 20 minutes, though it can take as long as a day.
The disconcerting part is that there's a seemingly-random mix of internal and external content. For example, searches find both everything on my rather large Twitter Timeline and my Twitter Direct Messages, which are a bit more private. Greplin doesn't find my Enterprise Search Engine Professionals group posts in LinkedIn, which is where I spend most of my time. And that wild directory project I worked on three years ago in Google Docs, which I don't actually want to see right now. And there's no way to control those things, as of yet.
The API, "coming soon", may solve some of these problems, if it allows for additional clients. I hope it also has services to make other social networks indexable, so that people can track their own posts on WordPress, BlogSpot, LiveJournal, etc.
* free service: the paid version currently also indexes EverNote, Yammer, and Google Apps Docs, Calendar, and Mail. They're also working on Dropbox attachments, Salesforce, Box.net, Basecamp, and Google Voice.