How to view Facebook in landscape in my new iPad Air?

To switch between Portrait and Landscape view, you generally only need to rotate the iPad to the desired screen orientation.

For iPad models without a side-switch, you may need to disable your Screen Rotation Lock. To do so, you need to access the Control Centre; swipe downwards from the top-right corner of the screen - then tap the Screen Rotation lock icon:

If screen rotation proves to be problem with some Apps - and not others - you need to be aware that not all Apps are coded to operate in both Portrait and Landscape screen modes. Apps that have a fixed screen orientation will always* appear in the orientation determined by the App Developer.

(*) For compatible Apps that are intended for iPhone, iPadOS15/16/17 will now automatically rotate the App to display in Landscape screen orientation. There is no manual configuration or setting to enable this feature.

However, Apps designed for iPad that are expressly coded to force Portrait screen orientation, iPadOS15/16/17 will honour the intended Portrait mode - and will not override the App coding.

For iPadOS16/17, where Apps explicitly force Portrait view, there is workaround. If your iPad model supports Stage Manager, portrait Apps can be displayed in Landscape orientation.

• iPad Air (5th generation and later)
• iPad Pro 11-inch (all generations)
• iPad Pro 12.9-inch (3rd generation and later)

Settings > Home Screen & Multitasking > Stage Manager > Use Stage Manager on iPad - set to ON

it should do that by default when you hold it in landscape unless it set to rotation lock

Change or lock the screen orientation on iPad - Apple Support

of cause you can also just try to restart the ipad

Restart your iPad - Apple Support