Board Thread:General Discussion/@comment-34234929-20180129224141/@comment-34193631-20180130023126

If you go into your friends list and click on their picture it will list that person's stats. Level, number of friends, collections completed etc. That's how that person would have known. You can also click on the icon with the two heads, then the 1 symbol, then 'only friends' box to see where you rank among your friends in different categories including number of collections completed.