Facebook’s plans for ‘Project Spartan’ have been rumoured for months: an extension of its web apps platform to smartphones and tablets. Contrary to expectation there was no announcement at Apple’s iPhone 4S event last week. Instead, the social network has unveiled its plans today.

“Today, we are extending Facebook Platform on mobile, bringing all the social channels that have helped apps and games reach hundreds of millions of users on the Web to mobile apps and websites,” blogs Facebook software development engineer Luke Shepard in a post aimed at developers.

“You can now easily reach the 350 million people who use Facebook every month on a mobile device, including iPad, iPhone, iPod touch, and our mobile web site.”

Shepard warns that it’s early days for this, with features still under development, and plans to extend to Android and other mobile platforms “in the near future”.

The move involves bringing features around applications on the Facebook website to its mobile apps, including bookmarks, requests, tappable links in newsfeed stories, and Facebook Credits for mobile web apps. Note the specific wording in that latter point: “Facebook Credits are not allowed in iOS apps or mobile web apps that are running within a Facebook iOS app.”

According to Shepard, around half of Facebook’s 350 million mobile users are accessing the social network through its apps, and half through its mobile website.

“Social apps are all about interacting with your friends so the best social apps will be on both native iOS and web apps, and we encourage you to think about how to reach the total audience of Facebook users by building both,” he writes.

Some developers and publishers got early access to the new platform, including Audiovroom, Branchout, Flixster, Gilt Groupe, and Huffington Post, as well as social games firms EA, Moblyng, Storm8, Wooga, and Zynga. Their initial apps can be tested by visiting http://fb.me/mobileappshowcase on a smartphone or tablet.

The news comes as Facebook finally releases its long-awaited native iPad app tonight. It has a big emphasis on swiping controls, full-screen photos and a seemingly carefully-planned navigation structure. It will also tie into the new mobile platform, playing games and using apps in full-screen mode.

This is all big news: essentially, Facebook wants anyone developing apps for its website to also be making them run on mobile phones and tablets. That in turn is a big boost for the HTML5 development standard, which means some serious questions to be asked within any company that has so far relied solely on Flash to make its Facebook applications.