30+ years in marketing from the trenches to the cloud. Digital focus, traditional principles. Business model comes first. Produce and host radio show “Minding Your Business" -- The Voice of Seattle Small Business, and author of “Social Persuasion” -- marketing and social media strategy for small business. MBA.
This 2012 study from Deloitte may give you some insights for extrapolating an estimate: http://www.deloitte.com/assets/Dcom-UnitedStates/Local%20Assets/Documents/us_chs_2012_mhealth_HowMobileTechnologyIsTransformingHealthCare_032213.pdf
For example, 247 million people have downloaded a healthcare app. 60% were for weight loss and exercise. Of the 13, 600 downloaded for iPhone, 5.3% were for chronic conditions. "Mobile technology supports increasing consumer engagement in wellness, prevention, and treatment programs, as well as chronic disease management. Pilot studies in diabetes and hypertension management have shown improved outcomes through remote monitoring."
As always, marketing strategy starts at the top with product, distribution, price, promotion, customers and competition. A successful promotional strategy is based on positioning, messaging and branding, as applied to advertising, PR, personal selling and sales promotions. Then, you need to understand these components before you choose the communications vehicles and know what to say to compel visitors to take action.
It's difficult to build awareness with social media alone, depending on the product or service of course. You may want to try social media as part of a broader PR campaign to drive people to a specific landing page at your website, with the call to action to provide an email. Be sure to give a good reason why they should in relation to your offering's ability to fill their need competitively.