Perhaps Alaska Airline’s spiraling stock price has provided the impetus for its newest online investment partner — ShareBuilder.

Mileage Plan members will receive 2,500 miles when they open and fund a new Mileage Plan ShareBuilder account, and an additional 1,500 miles each year on an active account’s anniversary date, up to a maximum of 10,000 miles. Unlike other online investment companies that have aligned themselves with frequent flyer programs, there is no account or investment minimum and no annual fee.

Interested members can go to http://www.alaskaair.com and click on the “Promotions” link to begin an automatic investing program. Real-time trades are $15.95, and you can pay $12 per month for unlimited “All you can Build” stock purchases.

Sharebuilder, with no annual fees and no minimum investment, is a welcome addition to Mileage Plan. Frequent flyer investors are a huge market, as evidenced by the American AAdvantage Money Market Fund, which reached $15 billion in early March. That figure is just under half of AMR Investment’s total assets under management.