Is there a Tweetdeck replacement for mac? A program that'll allow me to post to both my Twitter account and Facebook accounts with a single action?
I'd probably check out Socialite ($20).
Otherwise, you can use the official Twitter client and configure the Twitter Facebook App to post your twitter updates on facebook.
The latest Twitter API terms of service specifically prohibit clients posting to multiple social networks with a single click
5E: You may not use Twitter Content or other data collected from end users of your Client to create or maintain a separate status update or social network database or service.
So, even if clients like Hootsuite allow it now, I would expect them to not be able to do that in the future.