Is there any mobile app that doctors uses it frequently?
That depends on their need. Mostly in digitalis-ed hospitals they use HIMS, LIMS , Health Record Maintenance and Access app. There are also apps that help in data collection and recovery, such as entering information in the EHR or EMR of a patient. Communication and advice.
I see all doctors very active in Whats app, apart from personal use they use for thier professional work like interacting with their patients , checking medical reports , and groups discussion with peers.
Healthcare management professionals also use whats app, Linkedin is another app use by most of healcare management professionals.
PACS is also one of the commonly used app along with whats app