eg. I want to know if a user becomed online.

There will be an e-mail notifications page/tab on edit profile page.

I'll select some users one by one defining a period (expiring time) (easy select from my friends as an option) on this page.

If this user(s) becomes online in this period, system will send mail me. But if you make "hidden profile" feature, it won't send mail for hidden profiles.

Thanks.