As it turns out, not all text-messaging services are created equal. The difference between Facebook messaging from your cellphone and texting through the phone's built-in service may seem minimal to most of us, but our odd and archaic laws view them rather differently. As it turns out, it's very likely that your Facebook and Twitter messages are better protected from government snoops than your phone texts, even when you send them from the same device.
The actual message content enjoys protection in boths texts and social-media instant messages. That means the authorities need a warrant to read your missives, no matter how they're sent. But the metadata about your messages can be revealing, too. It shows who you contact, and when you do so.
Under one section of the law, 18 USC 2703(c)(1), the government can compel the production of internet communications metadata, such as the "to" and "from" information associated with emails, with either a search warrant or another type of court order (commonly described as a "d" order). This means that in order to obtain records about emails or Facebook messages, government agents need to convince a judge to issue an order compelling the production of those records.
Another part of the law, 18 USC 2703(c)(2), however, permits the government to obtain local and long distance toll billing records associated with the account with a mere subpoena.
In other words, the government can obtain a list of the numbers (and names) that you call with a subpoena, but finding out the names or email addresses of the people that you email requires a court order.
Phone companies already treat text message metadata in the same way they've grown accustomed to treating information about phone calls. That is, they only request a subpoena, not a warrant. Twitter and Facebook require warrants for that sort of information, whether you use their services through a phone or a computer. Google isn't very forthcoming about how it treats requests for users' data, apparently because it offers several services that offer similar functionality using a mixture of Internet and telephone technology. Google Voice, for example, seems to exist in a legal gray zone when it comes to Fourth Amendment protections.
So text away. Just remember that, privacy-wise, how you send that message really does matter, no matter how little sense that makes.