Getting notified about double-mile promotions on United

How does one find out about United's double-mile promotions? My friend who flies American says there's a secret email list she signed up for; I can't figure out from United's website how to sign up for such a list.

Here's what I've tried:

• I'm already subscribed to the "United News & Deals" newsletter, but it
  doesn't contain information on multiple-mile promotions.

• This page lists what newsletters you can opt into, and all I see is this under MileagePlus:
  
  • MileagePlus Partner
    Make the most of your membership — our wide array of travel, credit card and retail partners gives you hundreds of opportunities to earn miles on products and services you use every day.
  • MileagePlus Program
    Keep up with MileagePlus, the world's most rewarding loyalty program. Learn more about how you can redeem miles for award travel, upgrades, merchandise, once-in-a-lifetime experiences and more.
  • MileagePlus Statement
    Subscribe to My MileagePlus to stay up to date on your account and get personalized information and offers that fit your interests.

Thanks for any help!