Can someone tell me why my iPad 1 keeps returning to desktop whilst using Facebook?

Try resetting and restarting your iPad. Also try closing all other apps when you use FB and see if that helps.

If not, it might just be a sign of your iPad's hardware limitations. iPad 1 has too little RAM for some apps and websites now.

iPad 2 has twice as much RAM as iPad 1, and iPad 3 and 4 have twice as much as iPad 2.

Older iPads are going to increasingly crash as newer firmware, apps and websites take advantage of the improved hardware in newer iPads. That's the way of tech.