October 28 must have looked good on the PR calendar. A year and a half after the Facebook Platform launched to hundreds of indie developers and entrepreneurs in San Francisco, Yahoo and LinkedIn each made major platform announcements today. In short:

• Yahoo announced the launch of the Yahoo Social Platform and Yahoo Application Platform, major components of the Yahoo Open Strategy. As of today, the sandbox for developers to begin building OpenSocial applications on Yahoo is now open. While specific integration points and launch dates for Yahoo apps have not been announced, Yahoo says, “We will keep you updated on the rollout schedule for embedding third-party applications within the pages of Yahoo!’s properties.” The platform is fully internationalized from the start for 31 countries.
• LinkedIn announced the launch of the LinkedIn application platform, also built on OpenSocial, that will allow application developers to integrate apps on the LinkedIn home page and profile pages. 9 applications are part of the initial app directory, including apps by SixApart, WordPress, TripIt, Amazon, Box.net, Slideshare, Huddle, and Google. LinkedIn is taking a more controlled approach to app development, focusing on productivity and communication above entertainment. Here’s the launch commercial:

As more Platforms launch, application developers focused on different markets will begin to focus their energies on different platforms. We’ll be tracking which Facebook developers build for Yahoo and LinkedIn, and whether any nimble developers clone Facebook apps as they did on MySpace. Which Facebook apps do you think would do well if ported and adapted for Yahoo or LinkedIn?