In complete contrast to the opt-in sharing model, with the opt-out model, any user of the service will share everything by default. The amount of data that is shared, and with whom, depends on the needs of the service in question.
If you are not sure whether a social network or application that you are using (and have provided personal information to) uses an opt-in or opt-out model, it is best to assume that it uses an opt-out model. You should check the privacy and sharing settings of any online tool that you use to ensure that you are sharing only the information you wish to.
The benefits of this type of model definitely favor the service, not the user. Many users will never bother to view and edit their privacy and sharing settings on a service, blissfully ignorant of what the service may be doing with their information. The one benefit to the user here is that sharing more information may, and often does, make the service that she is using more valuable and useful.
Using an opt-out sharing model, the service can instantly capture and share all of the information that it needs from the user without having to prompt her or wait for her to enable the appropriate setting. This approach increases the service’s adoption rates, since more information is being shared and interaction between users will likely ramp up.