Thing is, you don't need an opener if you already exchanged messages.
What I do is message first, get IM address by second or third message, then start IMing right away at the first opportunity. Then all I start with something along the lines of "Hey! So let's get to know each other a little bit, m'kay?

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Nothing fancy, just set the mood of the chat. If you start gloomy, the chat will be gloomy. If you start with high energy and a fun attitude, she will copy that. And since chatting with you made her happy, that's bonus points for you. Then go on with normal get-to-know-you routines, name your five best traits, etc. Make sure to make fun of her sometimes, joke around, and don't fuck around IMing too long.
Also, if it helps, my standard first message consists of:
-A variation of "Hey! You seem cute/cool/sweet. Of course, I could be wrong, and that smile/those looks are hiding a fericious chihuahua, who knows ;P" (use one that fits)
Then I say something about her profile. Something that caught my eye about it and I like, or that I miss her introduction, because you can get to know someone so much better by the way they write about themselves, or just say that she seems like someone I want to get to know better. Make sure to mention something that you like about them as a person, or it will seem like you just like their looks.
If they have enough info on their profile, I go for the MSN right away (Add me on MSN, Let's chat. My address is xxx@xxx)
But if they have no introduction ("About myself") or brief profiles, then I ask them to "write a bit about yourself, blow my mind

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Usually by my second message I get an IM address back.