I think of all of those WhatsApp has been the most successful both in terms of moat defense purposes and it still has the potential of turning into something like WeChat or LINE (or what they're doing with FB Messenger itself) with its own monetization.
For Skype I think it was mostly about rounding out the office suite, MS doesn't want anyone else getting a toehold in workplace collaboration and expanding from there - if you get real classical about it MS Office is the true DNA of the company even more so than Windows/infrastructure. Same reason they bought Yammer and why they were so early on in trying to move on Slack (with the $8B offer that Bill Gates intervened in and shot down).
For Skype I think it was mostly about rounding out the office suite, MS doesn't want anyone else getting a toehold in workplace collaboration and expanding from there - if you get real classical about it MS Office is the true DNA of the company even more so than Windows/infrastructure. Same reason they bought Yammer and why they were so early on in trying to move on Slack (with the $8B offer that Bill Gates intervened in and shot down).
[Edited slightly for clarity]