Is the wife considered divorced if the husband reactivates an old, almost abandoned Instagram account, given that they had agreed the wife would be considered divorced if the husband created a new account on any social media platform?

As long as you have not created a new account, the condition for the divorce to occur has not been met, and no divorce counts against you by opening your old account, unless you intended by this condition to refrain from opening any other account, whether new or old. In that case, your divorce occurs by opening this account.