Through Facebook, Twitter, Email, tumblr, whatever medium you wish.
Then post what you told them, and a response if you get one.
I said the following through a Facebook message:
"Hey guys, keep up the great work! All the latest videos and interviews have showed off a really great product! Looking forward to seeing more!"
And included this link: http://www.youtube.com/watch?v=BmD_zrdJGxk
No response yet, but I'll post if I get one.
Also the title may have been a bit ambitious. I don't need/ expect that many people to do this, but if you've got some genuine positive words, I know they'd love to hear them.
Especially with the recent heat they've gotten with leaks and beta organization from things that really weren't their fault.
I think they recieved it but are problaby to busy to respond back,.Its nice to send them thanks for making this game, instead of asking for more things. Kudos