I'm surprised no one else suggested this, but maybe telling them you want to get to know them isn't enough. I have been on dating sites in the past and when I would receive a msg like that, I was never really sure how to respond. "Sounds great! What do you wanna know?" was kind of my standard response b/c I didn't like being made to feel like I needed to initiate the conversation/getting to know them.
So, instead, why not pick something on their profile that interests you and say - "I really liked your profile, especially what you wrote about 'XYZ'. How did you get involved with that? (or insert appropriate question pertaining to topic here)."
That will open the doors to communication a little better and get a conversation going faster, IMO.