Then they're not installed according to the market. Just because they came pre-installed on the phone doesn't mean the market knows you have them, hence no update option.
Easiest way to do it is install the apps from the market, it will then see you have them already & replace with the market app.
After that you should get update messages when the new versions are put on to the market. These will now be listed in your downloads too.
I don't use twitter, facebook etc, but I can see that the versions i have on my phone are not the latest versions.
The market only knows what you've downloaded from it, not what's on your phone. This is not specific to any handset, but how it works for all android devices. But yes, I do have the dhd.
Some apps will check for updates upon launch, such as maps, but others don't, so the only way it knows is from what you've downloaded from the market already.