Like other text-messaging services, HeyWire gives you a real phone number to use in order to send and receive text messages. Messages that are sent also sync with the company's iOS and Android apps, so you can read and respond via multiple devices.
There is no per-message charge, but it does cost 20 Facebook Credits per month to hook you up with a U.S. phone number and that unlimited texting access to any mobile phone in the U.S., Canada, Mexico and China and to most phones in the Caribbean and Central and South America. (The iPhone and Android apps are free, but are ad-supported).
While 20 Facebook Credits per month isn't a huge fee, it may be a big hurdle for some people to overcome in order to use this service. After all, if you're on Facebook, you might just chat with people using Facebook Messaging rather than sending an SMS. Of course, there are a few folks who still aren't on Facebook.
HeyWire recently partnered with Twitter to launch HeyTweet, a service that lets users send tweets for free via SMS. That service seems a lot more practical than this Facebook app.