I think fewer people leave their IM app on push 24/7 whereas Android users typically have gtalk on 24/7. It's no longer just a work chat thing people do by having gmail secretly open. It's now the best way to get ahold of them on their phone besides texting.
You have to admit Gtalk is treated a little differently on the iPhone.
My point was: his friends are going to have to install a special app on their iPhones anyway, regardless if it's GTalk via Beejive/IM+/imo, or Whatsapp/Kakao/etc.
I'm thinking the OP either:
1. doesn't realize that you can have GTalk on iPhones, or
2. doesn't realize that for this particular usage his friends will need to install a special instant messaging program on their iPhones (and keep them on push 24/7)
At the end of the day I think it really doesn't matter what we think is better (or how 'big' something is in Asia or whatever). If the OP's friends don't want to bother installing some strange app that they only need to communicate with him, it's not going to work. My suggestion is for the OP to ask his friends what *they* are using for cross platform messaging, and install that on his android phone.