Texting (SMS) is fairly universal though, no? I live in the US so I have bias based on that, but ever since the advent of texting I haven’t understood the need for a limited instant messenger. Why not just text everyone? And that’s where iMessage comes in handy. Whether or not you have an iPhone I can message you the same way. If you have an iPhone then we get all the benefits of iMessage. If you don’t, we can still communicate just fine. No need to configure multiple different applications to communicate with different people. It’s already built in to your phone number… why complicate things?
If you really want to have a more robust communication platform that can be used from PCs and whatnot as well as your phone, then use email, which again is universal and works across all different preferred email applications. Don’t understand the desire to gravitate towards a limited proprietary alternative to what everyone already has.