I saw a poster (http://staff.science.uva.nl/~kbalog/files/eacl2006-moodsignals.pdf) at a conference last year which used LJ mood tags to attempt to explain "spikes in the mood of the blogosphere". They trawled LJ to find days when lots of people wrote posts tagged "excited" or whatever, and then searched Google news to try and find an explanation (e.g. release of new Harry Potter book).