Even though it may appear that a user is no longer updating their journal, this can be misleading; the user might be making Friends-Only posts, or simply use the account for reading friends' posts and commenting on them. Some users also find themselves without Internet access for prolonged periods of time, but will continue with their journal at a later date.
InsaneJournal normally would not delete a journal for inactivity. However, due to the large amount of users that have migrated to the site in comparison to the number of inactive journals from earlier years, site administration has decided to purge older inactive accounts. More information on this process can be found in the recent announcements asylum at the following link: http://asylums.insanejournal.com/announcements/43568.html
A site purge is a rarity and will not be performed often.