If you use a GPL library all of your code has to be released under a GPL compatible licence. The Meteor framework counts as a library due to the way it interacts with your code.
You might be confusing it with the Lesser GPL which allows library code to be incorporated into propriety software (or websites in this case).
The way Meteor works is that the client side framework is bundled with your code and pushed to the browser of the end user, it's this pushing of the merged framework + your code that counts as distribution under the GPL.
The code isn't just run server side.
Also you should read the following sections of the GPL faq which should help clarify your understanding of the issues:
What I don't understand well yet is the way Meteor works. I'm waiting until they (and firebase) make their final version available, although I've played a bit with them.
If it does bundle itself then it could have implications. Maybe LGPL would be more reasonable then.
If you use a GPL library all of your code has to be released under a GPL compatible licence. The Meteor framework counts as a library due to the way it interacts with your code.
You might be confusing it with the Lesser GPL which allows library code to be incorporated into propriety software (or websites in this case).
To understand the difference between the two you should read: http://www.gnu.org/licenses/why-not-lgpl.html