By default, when you open the MySite SC for the first time, you will receive this dialog:
So, probably under your unknow of SharePoint, you will press “Okay”, but what you don’t know is that pressing that “Okay”, changes the privacy of all your social activities to “everyone”, and also send you an email. So probably, most of users don’t want to share their activities, but what we can do to solve this? PowerShell of course:
$web = get-SPWeb http://urlmysitehost
Hope that helps!