  We investigate the timing of messages sent in two online communities with
respect to growth fluctuations and long-term correlations. We find that the
timing of sending and receiving messages comprises pronounced long-term
persistence. Considering the activity of the community members as growing
entities, i.e. the cumulative number of messages sent (or received) by the
individuals, we identify non-trivial scaling in the growth fluctuations which
we relate to the long-term correlations. We find a connection between the
scaling exponents of the growth and the long-term correlations which is
supported by numerical simulations based on peaks over threshold. In addition,
we find that the activity on directed links between pairs of members exhibits
long-term correlations, indicating that communication activity with the most
liked partners may be responsible for the long-term persistence in the timing
of messages. Finally, we show that the number of messages, $M$, and the number
of communication partners, $K$, of the individual members are correlated
following a power-law, $K\sim M^\lambda$, with exponent $\lambda\approx 3/4$.
