Why is my iPhone 4s battery life still poor after a swap?

My iPhone 4S used to easily last me an entire day of listening to music and periodic texting, Facebooking, emails, Web browsing, etc. However, for the last couple of months, I've needed to recharge it for at least short periods to make it through the end of the day.

I swapped the battery with a new one from iFixit — no issues in the process — but it the problem seems as bad or worse than before. My battery life dropped from 100% to 60% after about an hour and a half of listening to music. I've also seen it plunge 10-15% in a matter of minutes lately.

I've turned off WiFi when not at home, turned off Bluetooth, closed down my apps, kept my screen as dim as my eyes can handle, but nothing seems to help — except for not using it at all, which kind of defeats the purpose. As I'm writing this (not from my phone), I've listened to some music and sent a few texts, and I'm down to 89% after 40 minutes. Make that 88%.

Is it a defective battery? Is there some sort of short in the system somewhere?

Also, I'm still running iOS 6.1, so it's not related to a software update. Now 87%. Wait — 86%.

Any thoughts?