When it comes to services, Apple is in a better position to partner than any other tech company. It's difficult for Facebook or Google to closely partner with other service providers because other providers steal user attention and hurt ad sales. Horizontal service companies trying to be on all devices depend on the user's choice of their services over other services.
A vertically integrated player like Apple doesn't care if its services dominate. Apple wants its services to be attractive and promote ecosystem stickiness, but Apple doesn't need its services to dominate to drive profits. That's because Apple's profitability is driven not by services, but by a three part mix of hardware, OS, and ecosystem. No single element of this mix dominates, and Apple can have a great ecosystem with in-house services/apps like contacts, calendar, notes, maps, and iWork, and third party services/apps provided by IBM, Microsoft, Twitter, Google, Facebook, and thousands of independent developers.
If you don't need your services to dominate or even be profitable to prosper, then you're free to partner with any service company you want. And that's what Apple does, partnering with IBM for enterprise software, Microsoft for its Office suite, and even Google for maps. These kinds of partnerships are difficult if you have a horizontal business model and you need your services to dominate to drive subscriptions or ad sales (e.g., Google search, Facebook, Evernote, etc.).
In a 2007, D5 interview Steve Jobs said he admired Microsoft's ability to partner with other companies. Apple now seems to address this issue by partnering as convenient on services while still owning the key hardware, manufacturing, and OS technologies and IP. See post titled Why Apple "Outsources" Applications and Services.
The author owns stock shares of Apple.