Both Weasyl and Deviantart have sections in their ToS that state "[we] reserve the right to terminate or suspend accounts that are inactive, in [our] sole discretion, for an extended period of time (thus deleting or suspending access to your Content)." in regards to vacant accounts.
I am especially interested in this policy also being implemented into FurryNetwork's ToS, as the issue of people signing in under a particular name and then leaving the account entirely vacant forever after seems to be a common one. This can be especially frustrating when other users would love to acquire those inactive account names.
Customer support service by UserEcho