How can I communicate with someone that only has a tablet?
Here's a couple of answers to start with:


Email
Hangouts (or any other IM app), which usually require the other person to be logged in.
Glympse - Send a check-in glympse to their mobile number with a message. They'll get it as a text.
using some SMS gateway either via Web (browser) or a specific SMS app supporting this
using a messenger app like Tango, Skype, or LINE