This is very interesting - I have been planning on implementing this in exactly the "wrong" way the article describes:

    Even if I was interested, sending a notification to me when they have
    joined is a pointless milestone. All you are telling me is that if I click,
    I get to see an empty profile.

So, are there any actions that make sense for the "FB friend joined" milestone? Prompting the user to message their friend in welcome seems artificial and annoying.