They should never have needed to even consider buying WhatsApp...I will never understand why Microsoft killed of there Messenger platform. I guess you could say because they bought skype and didnt want fragmentation, but skype served an entirely different use case. When it was still around I had the IM app on my phone and it was fine. Expanding it to lower end phones and supporting the social community would have done it.
--EDIT-- Microsoft reported the service attracted over 330 million active users each month, placing Messenger among the most widely used instant messaging clients in the world. [1]