Google has apparently backtracked on the idea of allowing third-party messaging apps to access RCS on Android phones. Though RCS is a public specification, APIs are needed to connect messaging apps to the RCS protocols employed by wireless network operators. No API means no RCS for outside apps.
Our API directory now includes 239 messaging APIs. The newest is the Parseco API. The most popular, in terms of mashups, is the Twilio SMS API. We list 169 Twilio SMS mashups. Below you'll find some more stats from the directory, including the entire list of messaging APIs.