  User activity fluctuations reflect the performance of online society. We
investigate the statistical properties of 1-min user activity time series of
simultaneously online users inhabited in 95 independent virtual worlds. The
number of online users exhibits clear intraday and weekly patterns due to
human's circadian rhythms and week cycles. Statistical analysis shows that the
distribution of absolute activity fluctuations has a power-law tail for 44
virtual worlds with an average tail exponent close to 2.15. The partition
function approach unveils that the absolute activity fluctuations possess
multifractal features for all the 95 virtual worlds. For the sample of 44
virtual worlds with power-law tailed distributions of the absolute activity
fluctuations, the width of singularity $\Delta\alpha$ is negatively correlated
with the maximum activity ($p$-value=0.070) and the time to the maximum
activity ($p$-value=0.010). The negative correlations are not observed for
neither the other 51 virtual worlds nor the whole sample of the 95 virtual
worlds. In addition, numerical experiments indicate that both temporal
structure and large fluctuations have influence on the multifractal spectrum.
We also find that the temporal structure has stronger impact on the singularity
width than large fluctuations.
