you can text out, as long as you're on Postpaid, not Prepaid. You can use WiFi to reach Facebook/Twitter/anything online, even if your phone is in airplane mode.
Texting from the US will cost you $5/100 outgoing texts, and you have a month to use up those 100 texts. Incoming texts are free.
if you're going to be around WiFi on a constant basis, try an app such as WhatsApp or Facebook Messenger (there are others, I don't know what they are) to call/text over WiFi