When using the vast array of 'free' Internet service users rarely stop and think "who is paying for this service?" there is an immense cost to providing an online service especially one that is used by millions of people.
Initially the cost is borne by investors that are sold the proposition, however over time they are going to want their investment returned with margin, and a good amount too. So once established with a large user base funded by investors, the tool may introduce a paid for premium service e.g. Dropbox with its larger space sizes, or Linked in with its premium features. Advertising will almost always be added to bolster the revenue stream. However more likely, real customers will be found – Marketing Agencies.
Many users feel the need to share their whole life on Facebook and twitter and this information is gold dust to marketeers, knowing vast amounts of information about someone (even if anonymous) enables marketing companies to build complex profiles of age groups, genders, communities etc., especiallywhen coupled with the ability to target adverts back to them that are guaranteed to be appropriate to the user, and their friends. The possibilities are endless, and the underlying motives become crystal clear when you look at the offering and ask yourself "who is the customer?" as it probably isn't you! You are the product.
This isn't a reason to shun such services, but to just stop for a minute and think about whether you are comfortable with what will happen to the information you are about to commit to the service.