You should try Moped Messenger. It is available on web and mobile and has all sorts of nifty integrations like venue sharing and cc'ing. Unlike whatsapp it is not tied to a phone number so you can use it on many devices at once. There's not an official iPad app yet, but the iphone app works scaled up and i've heard a proper iPad one is in the works...
Although not everyone is on moped yet, it degrades very nicely to email so that you can message anyone in your address book--they don't even need to signup to moped to interact with you.
