Money goes around. Remember that they give you money because they earn something from what you do. If you put up ads, they earn money from people clicking them and they give you a small portion of their earnings. They don't offer these pay programs without themselves earning money in some other way from you.
If someone visits your site, you earn something from it. Visitors are an important part of your site. Without them, you can't be called a true website and just be a clump of HTML files floating around in cyberspace. If someone were to pay you for impressions, ask yourself, what would that person benefit from you to make them pay you? Do they ask you to put up some kind of code in your site? An ad, perhaps? If you doubt that they'll earn something from it, doubt even more. It could be a scam, or a company that probably won't last. Ask around, you'll never know what may happen until it's too late.
Personally, I don't think that there's such a program that pays per impression. If there is, treat it with all the critisicm you can to find out if it's true. It's better to be a harsh critic than to end up losing all your hardwork in the end. People benefit from you. Always think about that. They don't give stuff to you for nothing.

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