I'll join in with f4780y in recommending RetinaPad. As much as I hate to admit it, Facebook has become necessary for me to keep in touch with a group of [much younger] family members. So, the iPhone Facebook app was the first thing I tried FullForce on. Blech! It looked like ... well, not good.
RetinaPad, while indeed a paid tweak ($2.99 USD) makes the Facebook app look as if it was designed for the iPad.
The difference was night and day. So, if you want a nice experience with iPhone apps, get RetinaPad. I've not found an iPhone app yet that this tweak doesn't work great with.
Standard caveats apply, so YMMV.
Marilyn