It's not a matter of what you can or cannot do. Tweetdeck can integrate with Facebook. Can Tweetie?
Loren has actually stated officially that Tweetie will (fortunately) stick to supporting twitter and only twitter, so as to provide the best experience in a singular area.
As for the whole dock icon removal thing, I've been begging for an update to allow dock icon removal for a long time. There was a HUGE thread about it on Get Satisfaction where 500+ people wanted the dock icon gone (or, conversely, the menu icon gone), and Loren's response was that there's no way to do this on OS X without sacrificing the menu bar.
Since he believes that the menu bar is "the heart of Tweetie", the dock icon isn't going anywhere. It's for similar reasons that RealMac removed LittleSnapper's option to hide the dock icon.
Personally, I could care less about the menu bar - give me my dock space back, please!
Founder of GoodSemester and ultra-webmaster of Zelda Universe and Zelda Wiki. I also have a blog that sometimes has ice cream.