You should always beware of free services. If they’re not making money from you, they are making money some other way; possibly by selling you (your data or access to you) to others (e.g. data mining companies, advertisers, etc.).
Jojo Hedaya, CEO of Unroll.me, who was exposed for selling user’s emails (ostensibly “anonymized”) in the recent New York Times piece about Uber’s CEO, seems trite and insincere in his response:
Our users are the heart of our company and service. So it was heartbreaking to see that some of our users were upset to learn about how we monetize our free service.
And while we try our best to be open about our business model, recent customer feedback tells me we werent explicit enough.
If you don’t know how the free service you use pays for itself, don’t use it. Odds are your data is funding the continued free service.