There's a local one which a lot of people already have installed even if they don't use it most of the time. A lot of people have Telegram installed too. WhatsA

There's a local one which a lot of people already have installed even if they don't use it most of the time. A lot of people have Telegram installed too. WhatsApp / FB Messenger are the most popular but if they're down, between those two you would have most people covered.