It?s up to the individual app developers to update their apps. There are a number of things to be updated this year. First there?s the iOS 8 updates. This includes UI changes as well as internal API calls. Then there?s the screen size changes required to support the iPhone 6 and iPhone 6 Plus native resolutions.
Some app developers are better than others at turning this stuff around. Some are notoriously slower. Since Instagram is now a Facebook thing, it probably will be severely lagging in the update department. Even though multiple recent updates have come, there is nothing to indicate that any of the FB apps have been updated to take advantage of anything offered by either iOS 8 or the new screen sizes. That?s pretty much par for the course, as far as FB goes. I deleted my account about 6-8 months ago, and honestly, I don?t miss a thing about it.
Several new updates came out over the past couple of days - Tweetbot, ESPN SportsCenter, the Apple Store, Fantastical and others. The new iOS and the new devices have only been out 7-10 days now, so just give it time. Many of them will start pushing their updates soon.