If you only need Internet access occasionally when you check Facebook, keep the connection off when you're not using it. You can use the drop-down notification menu (slide down from the top edge) to quickly turn it off. Keep Wifi, 3G, Bluetooth and particularly GPS off when you don't specifically need them. When you do check Facebook, use the Facebook For Android app, as it seems to be light on the resources and I personally have found it much better than browsing FB through Opera Mobile or the somewhat lackluster default browser.
If you send and receive a lot of texts then set the display lighting to the dimmest possible setting (which I personally find to be quite sufficiently lit) to save quite a bit of battery. Also set the phone to automatically turn off the screen in a very short amount of idling (as opposed to a minute or two).
I'm wary of making any promises as to what kind of battery mileage you're going to get, because it depends on usage, settings, and also how strong the network coverage is where you roam. I use 3G mobile broadband on my phone and I keep it on 24/7 so that I immediately get push alerts for new email etc. I also use Spotify to stream music over the same 3G connection and listen to it for a couple of hours straight each day. This is in addition to getting and making a few calls and sending a couple of texts, and a bit of Googling now and then, and checking FB. I have to charge the battery every night, and sometimes I use the car charger if need be during the day. This is with a constant day-long Internet connection, so I would expect you could hit two days without charging if your use is as light as you say, and if you take a bit of time to turn off everything you don't absolutely need.
Myself, I am a bit disappointed with the battery performance, but it's just a fact of life with smartphones, and the features and functionality for the price are amazing with Galaxy Ace...
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