I use to use an HTC Desire Z which is almost the same phone. I like it for the physical keyboard. The keys tend to be a bit stiff for my liking, but I got used to it after a while. The phone is almost 2 years old, so it is going to be slower than many of the newer phones, even the mid-range ones are probably faster thant he Merge.
For Facebook, most of the updates have been rather poor and is rather slow on my DZ. The latest FB update a couple of weeks ago was actually decent. If you do get the Merge, I suggest that once you find a decent version of the Facebook app to not bother doing any further updates unless you really want some feature in a new update.
For email, pictures and music, its OK. You may need to get more storage for this phone. On GSMArena, the stats indicate it only has 2GB of storage. For web browsing, it can be slow. The Opera Mini browser will improve the speed of web browsing, but there may be some content that you may not be able to see since Opera Mini is not a full function browser.
You should definitely try out a working model if possible before you buy one. Since this is an older phone, there may not be a working demo model at most stores.