We first wrote about HeyWire, a Boston based startup at SXSWi when it integrated Twitter, Facebook and Google Talk into its group messaging app. The app aims to be a truly multi-platform messaging service, but it couldn’t do that without a Facebook app too, right?

Today, HeyWire launched its Facebook App, which is the first app that allows Facebook users to send both texts and Tweets via SMS from within Facebook.

“HeyWire is all about providing better ways for people to connect around the world, so we’re excited to extend HeyWire’s Social Hub to Facebook’s 600 million+ users,” said Meredith Flynn-Ripley, CEO of HeyWire in the release. “When coupled with HeyWire’s iOS or Android app, users will have the added benefit of seamlessly carrying on any text conversation through Facebook online and on their mobile device while on the go!”

2 Dialogue 520x391 HeyWire now lets you text your friends from Facebook

Unlimited texting from within Facebook to any mobile phone in the USA, Canada, Mexico and China, and to most mobile phones in the Caribbean and Central and South America costs 20 Facebook Credits per month. Texts can be sent to your friends in any country, in most any language, if your friend is a HeyWire user.

If you download the iOS app, I particularly like HeyWire as it gives international residents a US number, so they never need to pay for a text or call to the U.S. again when hooked up to an Internet connection. Within the third iteration of HeyWire’s app, a user will be able to group text multiple mobile numbers, people in their Facebook chat or GTalk contacts and simultaneously update their status on Twitter by including “40404.” In fact, HeyWire claims to be the only app that lets users do this. Replies can be individual or to the group. So you can begin communicating one-to-many, receive messages back one-to-one (and keep messaging privately back and forth).

The app is free to use, so why are they charging for Facebook? It’s all about the business model says HeyWire. Texting is free within the app because it’s supported by advertising (and on iOS through in-app purchases). Within the Facebook App there is no advertising, so HeyWire is charging a small amount.

Interested in using Facebook for phone calls? Check out KipCall, an iPhone app which lets you call your Facebook friends for free, no WiFi or 3G needed.