When Steve Jobs first announced FaceTime for the iPhone 4 back in 2010, he said not only was it based on open standards, but that Apple would be releasing the FaceTime protocol itself as an open standard. That would allow third parties to create FaceTime clients for Android, Windows, BlackBerry, or any other platform. Now, 4 years later, there's still no open standard release in sight, much less cross-platform FaceTime clients. So, what happened?
The easiest, simplest explanation is that Apple has reneged on Steve Jobs' promise to release the FaceTime protocol as an open standard. That FaceTime proved too valuable to them as a proprietary implementation and so they changed their plans, and that's that. Anything is certainly possible, although my understanding is that the reason is more complicated, and far more maddening.
In August of 2010, roughly 2 months after the iPhone 4 was announced, Apple was sued by a patent holding company named VirnetX over a "method for establishing secure communication link between computers of virtual private network". Apple refused to settle, so they went to court. VirnetX continued adding services, including FaceTime, and devices, including the iPhone 4s and iPhone 5 to the suit. In 2012, Apple lost to the tune of $368 million. That might not sound like a lot of money to a company like Apple with over a hundred billion in the bank, but Apple was also set to pay millions more in ongoing royalties to VirnetX for a service they were essentially giving away for free.
Apple hasn't given up on FaceTime. Just the opposite. They announced FaceTime Audio as part of iOS 7 and continue to actively market it, releasing a FaceTime every day commercial just last summer. While still maintaining they didn't infringe on any VirnetX patents, Apple began re-architecting FaceTime to go through relay servers and, hence, work around the patent.
The FaceTime litigation is still ongoing, and VirnetX still believes Apple is infringing on their patents in general, recently adding the iPhone 5s, iPhone 5c, iPad Air, and Retina iPad mini to the suit.
How Apple originally envisioned third party FaceTime implementations working was never disclosed. It looks like Apple would have to handle device identification, or create a system that could handle both Apple-identified and third-party identified devices. What's certain is that Apple never expected to have to run all FaceTime calls through their own servers. Apple, historically, doesn't excel at internet services, and it's no doubt a strain on their resources.
Adding Android users under this model, where they'd have to go through Apple's relay servers, is no-doubt a non-starter. Were some other company willing an able to use their own relay servers, and able to tie into Apple system, they'd still be subject to the same litigation from VirnetX, who has previously sued Microsoft, Cisco, and numerous others.
I'd very much love to see FaceTime for Android, FaceTime for Windows, even FaceTime for BlackBerry and Linux. (I'd also still love to see FaceTime merged with iMessage. Given the patent trolling, given then relay servers, and given the reality of the world we live in, I just don't think we will, and certainly not any time soon.