Mine is the same and it's very frustrating. Facebook takes 20-30 seconds to load, everytime, and is then slow to navigate.
The app store is the same, not worth using. What's app too. I've plenty of space left for storage, am using 4.2.1 and have done a rest (home and power hold).
I feel learned the hardway that it really isn't a good idea to sign up for a two year contract with the iPhone and won't do it again in a hurry.
My phone is definitely worse now than it was when I bought it - and we're talking about old applications here, not ones that have come along recently
If you find an answer, I'd be grateful to hear it. I'd read elsewhere there might be an update, but it's the same old problem - we have to make do with a rubbish phone for a month or so until Apple acknowledges it as an issue and looks to do something.
We've been here before