Cross platform iMessage-like app.

With so many SMS alternatives, you would think this is an easy answer, but I can't seem to find a real alternative to iMessage.

Let's recap a bit. iMessage is a service provided by Apple that let you send text to your contacts via data, be it wifi/3G. The beauty of iMessage is one account will work and sync with all your Macs and iDevices. No need for separate accounts for each device. Downside is, it's Apple ecosystem only. I want something that I can use with all my iDevices and Android devices, and a PC/Mac/web client is a huge bonus.

The alternatives that I've tried:
-Whatsapp: One device only. You account is tied to your SIM/number. If you change SIM, you will have a separate account. If you have another device with the app, you have a separate account. No non-phone devices (there are some workarounds, but I'm not going there), and no PC app. Yeah, it's pretty popular, but extremely limited once you look at it.

-Kakao, Viber, all are the same. You cannot have one account syncing on multiple devices.

-Line: Also tied to 1 device/phone number. Although you can install the client on non-phone devices, if you log in on, let's say, a tablet, the next time you log-in on your phone, you have been logged out. If you have two phones, unless you like logging in and resetting your chat history every single time, it doesn't work. On the bright side, they have a PC app.

-Google Voice: is great. App can be installed on non-phone devices, things are synced just fine on multiple devices since the information is in the cloud, and you can access everything from the website. Even better, the other person doesn't have to have Google Voice. But it is US only, and no MMS.

Skype? I don't have friends on it, and I don't think it synced the chat history between devices. Plus the app seems "heavier" than the previous examples as it's a true VOIP client.

Do I really have to use Facebook?