22. The non-transitory storage device of claim 21 , wherein the activity information comprises: sensor readings of the first user device indicative of movement of the first user device; a history of content access that occurred on the first user device at the location; or a history of application program launching that occurred on the first user device at the location.
21. At least one non-transitory storage device storing instructions operable to cause one or more computing devices to perform operations comprising: receiving, from a first user device, activity information on an activity performed by a user of the first user device; receiving, from the first user device, a location where the first user device has stayed for at least a threshold amount of time; determining a type of the activity; creating a social group based on the location and the type of the activity, including determining a theme of the social group based on the type of the activity; determining that a first condition that a second user device is located at the location or will visit the location is satisfied; determining that a second condition that the second user device seeks information related to the location or related to the theme of the social group is satisfied; and in response to determining that the first and second conditions are satisfied, providing a recommendation to join the social group to the second user device.