First of all, welcome to the forum!
If you are registering a typo in .net of a site that is in the "top 500 sites" then you may very well be infringing on either a registered or common law trademark.
Regardless of whether "others" may be doing this, a history TM infringement can be used against you. The courts have repeatedly found that typo's of marks are infringement. While a few bucks from infringing typos can benefit for a short while, a bad reputation stays with you for life.
Marc