Twilert is like Google Alerts for Twitter. We track mentions and keywords on Twitter and email our users at a time and frequency they choose. Normally once per day. We have 40 000 users and more are being added every day. Most users track two or more searches on Twilert. We send 1.2 million emails per month (also growing steadily).
We include one advert per email prominently at the top and have a very high open rate as users want to read the content of our emails... they asked to receive it!