One of the main problems of carpooling is reliability, due to the fact that the service requires users to be confident that the driver will take them to their destination, and that drivers be sure that the accompanying people will behave properly during the trip. A practical improvement applicable to existing carpooling systems is described here, based on the use of the latest technological advances in smartphones and social networks to increase reliability. The proposed solution first allows the establishment of trust between drivers and passengers through reputation accounting, and, second, protects the privacy of all users. This is the main aim of this work, but there are others related to carpooling such as route optimization or enhancement of cooperation, which are not addressed here.