For remembering particular accounts (for example of friends), I want to propose a tool in which you are able to organize your "contacts". This could be a simple list or also a table..
The other account has to agree that it is one of your contacts, just like a "friend requiest" (like on geocaching.com).

-> I don't think it makes sense to add private message. Because my idea is to visualzie the contacts in a table, there should be also a column with the email-adress.