I am finally biting the bullet. I will start to really anonymize my graphs. In order to do so, I was trying to find a tool on the Web which does that. Well, as you can probably imagine, there is non which does exactly what I wanted. So i wrote my own anonymization script. To safe you some hassle, also download the Anonymous.pm file.
This is how you use the script on a CSV file:
cat /tmp/log | ./anonymize.pl -c 1 -p user
This will replace all the values in column one with usernames of the form: "userX". If you are anonymizing IP addresses, run the tool without the prefix (-p) and it will do that automatically for you.
Credits to John Kristoff who wrote the Anonymous.pm module for Perl.