If you're making those free users work for the value their receiving (shares, referrals, etc.), then I think it's fair. Yesware is phenomenal at this, they allo

If you're making those free users work for the value their receiving (shares, referrals, etc.), then I think it's fair. Yesware is phenomenal at this, they allow you to track opens on your sent emails, but their free plan only gives you a 100 tracks/month unless you share and get others to sign up. Dropbox is the poster child of this though (spoke about it below).