Diaspora — the project started as an open source alternative to Facebook at the time Facebook was facing user complaints to their changes to the user privacy — has published its first alpha version on GitHub. According to the README, it sounds like Diaspora is using MongoDB.
I am pretty sure that the decision was not made based on the recent MongoDB scaling features, but rather on the feature set that made the initial developers feel comfortable and familiar to develop this first alpha version. On the other hand, seeing Rails 3 in the same list may just mean they tried their hands with the latest and greatest.
Original title and link: Diaspora, The Open Source Social Network, Uses MongoDB (NoSQL databases © myNoSQL)