To make a long story short, it sounds like iOS 4 was supposed to include the type of Facebook integration that iOS 5 will enjoy with Twitter but Apple and Facebook couldn't get the deal done. Subsequent squabbles over Ping, and a tempestuous visit by Steve Jobs to Mark Zuckerberg -- including a blow up over HP and their TouchPad -- only fanned the flames of discontent. According to Jennifer Van Grove at Mashable:
When Jobs learned of the webOS Facebook app during his summer visit to Facebook, he was livid. Zuckerberg vowed to get the app pulled. But Jon Rubinstein, the former CEO of Palm and then the GM of HP’s webOS division, refused to halt the release of the app. Facebook responded by restricting HP’s access to its APIs — just as it had done with Apple’s Ping, a year earlier.
Apple wanted Facebook on iPad first, HP wanted Facebook on TouchPad first, and Facebook played both sides to the apparent benefit of... nobody. Well done.
For the whole, long, sordid version of the tale, hit the source link below. Meanwhile let's hope Apple and Facebook really have unburned their bridges and we get Facebook for iPad on or around this Tuesday as rumored.