I hadn't noticed. That's kind of strange, but whoever/whatever they are, they don't seem to be doing much:
Code:
day rx | tx | total | avg. rate
------------------------+-------------+-------------+---------------
09/07/11 32.17 MiB | 207.76 MiB | 239.92 MiB | 22.75 kbit/s
09/08/11 80.99 MiB | 245.05 MiB | 326.04 MiB | 30.91 kbit/s
09/09/11 72.91 MiB | 197.89 MiB | 270.79 MiB | 25.68 kbit/s
09/10/11 26.92 MiB | 170.02 MiB | 196.93 MiB | 18.67 kbit/s
09/11/11 25.68 MiB | 184.81 MiB | 210.48 MiB | 19.96 kbit/s
09/12/11 31.58 MiB | 217.12 MiB | 248.70 MiB | 23.58 kbit/s
09/13/11 40.04 MiB | 244.83 MiB | 284.87 MiB | 27.01 kbit/s
09/14/11 41.29 MiB | 255.71 MiB | 297.00 MiB | 28.16 kbit/s
09/15/11 90.90 MiB | 222.71 MiB | 313.61 MiB | 29.73 kbit/s
09/16/11 39.53 MiB | 253.12 MiB | 292.65 MiB | 27.75 kbit/s
09/17/11 32.69 MiB | 160.11 MiB | 192.79 MiB | 18.28 kbit/s
09/18/11 22.78 MiB | 155.15 MiB | 177.93 MiB | 16.87 kbit/s
09/19/11 39.47 MiB | 261.28 MiB | 300.75 MiB | 28.52 kbit/s
09/20/11 39.34 MiB | 260.53 MiB | 299.87 MiB | 28.43 kbit/s
09/21/11 41.84 MiB | 258.59 MiB | 300.43 MiB | 28.49 kbit/s
09/22/11 42.70 MiB | 320.36 MiB | 363.06 MiB | 34.42 kbit/s
09/23/11 34.71 MiB | 248.10 MiB | 282.80 MiB | 26.81 kbit/s
09/24/11 26.96 MiB | 240.72 MiB | 267.69 MiB | 25.38 kbit/s
09/25/11 27.85 MiB | 198.18 MiB | 226.03 MiB | 21.43 kbit/s
09/26/11 40.40 MiB | 273.24 MiB | 313.65 MiB | 29.74 kbit/s
09/27/11 50.51 MiB | 237.31 MiB | 287.81 MiB | 27.29 kbit/s
09/28/11 49.30 MiB | 291.32 MiB | 340.62 MiB | 32.30 kbit/s
09/29/11 50.42 MiB | 273.50 MiB | 323.92 MiB | 30.71 kbit/s
09/30/11 36.53 MiB | 258.05 MiB | 294.58 MiB | 27.93 kbit/s
10/01/11 26.37 MiB | 189.41 MiB | 215.78 MiB | 20.46 kbit/s
10/02/11 28.50 MiB | 215.87 MiB | 244.37 MiB | 23.17 kbit/s
10/03/11 85.69 MiB | 288.84 MiB | 374.53 MiB | 35.51 kbit/s
10/04/11 35.99 MiB | 252.06 MiB | 288.04 MiB | 27.31 kbit/s
10/05/11 41.44 MiB | 280.64 MiB | 322.09 MiB | 30.54 kbit/s
10/06/11 67.61 MiB | 171.77 MiB | 239.38 MiB | 36.73 kbit/s
------------------------+-------------+-------------+---------------
estimated 108 MiB | 276 MiB | 384 MiB |
Other than that weird incident in August, total bandwidth usage has been very constant.
It's possible that something has gone wrong with phpbb's session accounting, and anonymous sessions aren't expiring correctly. Debian (and by extension, Ubuntu) have a kind of weird mechanism for cleaning up the PHP session dir. Maybe the cron job is broken? Usually that results in email to root, but not always. I'll look into it.