Earning Money through internet advertisement and sharing it with the users.

I?m running an email service (dot com domain) and earning money from it through advertisement. As there are many websites on the internet who are giving free email facilities to their users like yahoo, hotmail and earning money through advertisement. So, in order to make my email service more attractive for the users I start a program known as earn from your email . User who register with our email service will get a free email account plus some earning opportunities. We get advertisement(those within the boundaries of Islam) who pay us money for promoting their websites. We then share this money from our users and give them 50% of our earning for registering with our advertiser. For example , advertiser abc.com ask us that they want 5000 users on their website and will pay 0.50$ for every user who register with their website. We then send an email to our users asking them that If they register with the website abc.com we will pay 0.25 $ to them. The user who in turn registers with the website abc.com by filling the online form will get 0.25 $ and the remaining 0.25 $ will be our profit. Is that way of earning money both for the company as well as for the users is correct according to Islam. ?