Google, Microsoft fight over standards to rival Apple's FaceTime
In May 2011, Microsoft acquired Skype for $8.5 billion to aggressively enter the video conferencing market. However, Skype is fundamentally, technologically different from FaceTime, using a wholly proprietary system of call setup, authentication and peer-to-peer transmission that is not even remotely compatible with FaceTime on any level. Trying to integrate the two would be as difficult as playing a Nintendo DS game cartridge in your car's CD player.
Microsoft already offers a third party iOS client for Skype (which existed before its acquisition), so both iOS and Mac users can use Skype, they just need to use a separate app to do so. More importantly, Skype users can't call FaceTime clients, and vice-versa.
Apple hasn't shown any interest in connecting to other non-standard video conferencing systems within FaceTime. It originally supported AIM's proprietary video chats within iChat AV on the desktop, but never extended support for AOL's proprietary video IM system to iOS. It has since lost interest in promoting interoperability with AIM users.
Apple also supported Google Talk within iChat on the Mac (via the open nature of XMPP/Jabber), but not for video conferencing from iOS. Google has also changed its strategic direction with Google Talk video conferencing, as noted below.
Google offers WebRTC as a FaceTime alternative
As with Microsoft, rather than expressing interest in FaceTime, Google has focused upon its own investment of $68.2 million in Global IP Solutions, which it announced the intention to acquire in May 2010, just weeks before Apple unveiled FaceTime. Google completed the acquisition in January 2011.
In contrast, Apple has never relied on the web to deliver video conferencing. It has always used on native apps, first iChat on the Mac, then FaceTime on iOS, and then FaceTime on Mac. Apple has since renamed iChat on the Mac to Messages, which it maintains separately from FaceTime.
Google took GIPS's voice-optimized audio codecs (iSAC and iLBC) and added its own WebM video codec (née VP8) to deliver WebRTC as part of its 2010 strategy to replace H.264 web videos with its own WebM codec.
While WebRTC can technically be made to work with any video codec, including H.264, Google's own clients are naturally designed to use WebM, so any client that wants to work with Google's would have to support Google's WebM. That's of course something Apple has no interest in doing, because its iOS devices have no support for WebM hardware acceleration.
Same same, but different
Technologically, WebRTC isn't nearly as different from FaceTime as Microsoft's Skype is. Both FaceTime and WebRTC are based on SIP for call setup, use RTP for video delivery, and rely upon ICE, TURN and STUN for handling firewalls and NAT.
However, Google's WebRTC is essentially an experimental open source project being offered to web developers, not a finished product like FaceTime. This makes it more akin to Google Wave compared to Apple's Mail app: one is a complex technology erector set, the other is an easy to use, finished end-user app.
Of course, the difference is that Mail can be configured to use Google's gmail accounts; there are no equivalent "video conferencing accounts" usable with FaceTime, which is currently hardwired excessively to Apple's authentication and push notification servers.
Given Google's interest in establishing WebRTC as a browser standard and its ongoing (if stalled) efforts to substitute H.264 with WebM, it does not seem at all likely that Google would be interested in working with Apple to develop licensed FaceTime clients for the web, Chrome OS or Android.
Conversely, Apple does not appear to be interested in working with Google to provide interoperability with iOS and Macs using FaceTime, given that it has taken actions to remove Google from iOS 6 Maps, drop its iOS YouTube app, and won't even natively support Google's social networks in OS X Mountain Lion's Share Sheets.
On page 3 of 3: FaceTime, WebRTC not directly comparable; Microsoft proposes CU-RTC-Web