iMessage came after WhatsApp (WA itself isn't the first messaging app...), I wonder why WA didn't get big in the US, maybe it's because virtually all mobile contracts in the US include a texting package. As for WA, one key to its success was probably the automatic upload of your address book to see who of your contacts is already in the system (and the privacy side in the battle of privacy vs convenience loses another battle), on other IM clients you had to add contacts manually.