I tend to agree with Laura, that six months is too long.
as far as I know though it should be possible to differentiate between
"deactivating" and "removing/losing/deleting" a profile.
By deactivate I mean something like "put into invisible hibernation"
after say 3 months of inactivity. That would mean that the profile is
still technically in the system, but doesn't show up in search results
or by clicking "random address". If the user had left a message on
someone else's profile and one clicked on the name, then instead of
the profile being displayed, a message like "user X has not logged on
for a while, so the profile has automatically been placed in
hiberation" or something similar. If the profile owner logged on
again after the three months, then the profile could be restored/woken
up automatically. Depending on whether hibernating profiles
eventually might become a problem, you could set a second "deadline"
whereby profiles which have been in hibernation for three months
(giving a total of 6 months of inactivity) would be deleted. But
that's step I think which would only be justified for technical
reasons, since for "live" end users there is almost no difference
between a hibernating profile and a deleted one.