As dleslie and that_guy_iain have mentioned, it was expensive to send an SMS text back then, especially in Europe when your friends have different country codes. So WhatsApp was a free alternative that tapped the unmet need for millions.
And signing up with only a phone number seems like a nobrainer nowadays, but the mindset of the time was still email + password. Sure, Microsoft could have launched a competitor, but they would most certainly require you to use an MSN account, thus adding friction to adoption.
As dleslie and that_guy_iain have mentioned, it was expensive to send an SMS text back then, especially in Europe when your friends have different country codes. So WhatsApp was a free alternative that tapped the unmet need for millions.
And signing up with only a phone number seems like a nobrainer nowadays, but the mindset of the time was still email + password. Sure, Microsoft could have launched a competitor, but they would most certainly require you to use an MSN account, thus adding friction to adoption.