  A sentiment index measures the average emotional level in a corpus. We
introduce four such indexes and use them to gauge average "positiveness" of a
population during some period based on posts in a social network. This article
for the first time presents a text-, rather than word-based sentiment index.
Furthermore, this study presents the first large-scale study of the sentiment
index of the Russian-speaking Facebook. Our results are consistent with the
prior experiments for the English language.
