Google's Accelerated Mobile Pages allow content to be served in a mobile-optimized fashion through Google's servers that can load up and be displayed in a matter of seconds, even on mediocre connections. Facebook's Instant Articles, meanwhile, while essentially the same thing, uses Facebook's servers as a backend, meaning things are more than a little different under the hood. Obviously, this means that even though the two different page architectures serve the same purpose for the end user, the two different backends mean that they're individually suited to different web developers based on a number of factors. While Facebook Instant Articles may implement painlessly with Facebook, for example, Google's AMP page architecture is open-source, allowing for wider implementation but at the cost of a bit more elbow grease. AMP has a few advantages that will likely appeal to a wider variety of developers, for the time being.
For starters, being open-source means that web developers using AMP can get just about any page and feature working, if they're willing to put in the backend work. Everything from HTML5 content to high-resolution images can be pushed through Google's optimization processes, and those processes can be tweaked for specific content. Savvy web developers can also fork the AMP architecture framework to make a personalized version for their site or content. Developers can also build upon AMP to create something entirely unique to the platform, like a hyper-optimized mobile experience or high-end content like WebGL. Add in the fact that AMP is a Google product and thus always in development, and you have the potential for some really great features to come to the platform. It also means that applications built for the platform will likely have a long shelf life, and bugs will be reported, logged, and squashed with each release. Facebook Instant Articles is also in development for the foreseeable future, but the API is closely tied to Facebook, somewhat limiting the platform's scope. On top of that, Facebook Instant Articles is not open-source, so developers can't work terribly deep within the stack, and certainly can't create a hyper-optimized version for their own content; it's a purely one-size-fits-all solution, though not a bad one at all for displaying quick and clean pages. Naturally, AMP is being worked on at a more frantic pace, and the platform is already compatible with ads, which is a major feature for such a platform.
On the user side of things, AMP articles are shown in a convenient carousel among search results, since Google Search is the jumping-off point for AMP pages, for now. Webmasters can, of course, integrate AMP into any of their pages and even their entire site, but the initial contact being through such a neat and intuitive interface does quite a lot to help the user experience. The carousel can also show similar pages to yours, meaning that users can grab a bit more info without having to Google around, once they finish reading your page. When it comes to products, this will often lead to a more informed purchase, and when it comes to most other kinds of content, this will often end with a user checking out some extras around the web, then bookmarking your page as the interesting starting point that they got all that info through. Finally, sharing is much more robust on the AMP platform. While the sharing options that users of Facebook Instant Articles are stuck with all point to Facebook in some form, AMP has access to a platform's entire sharing intent, and more different options can be added in by simply coding them in, since AMP is open-source. Users on both iOS and Android face the same limitations with Facebook Instant Articles, and the same flexibility with AMP. Instant Articles, however, do have a comment API, which is something AMP lacks, that gives them a decidedly social edge. Ultimately, it's up to the web developer to decide for themselves which platform better meets their individual needs.