Users of the smartphone app may be added into groups set up by people they do not know. The groups may contain data about them for sales promotions.
A user who has been added into a group said he was sent a message from a mainland number. He could also see the phone numbers of several local users through the group.
Lawmaker Charles Peter Mok said it was difficult for authorities to track down the senders of messages from the mainland. "Authorities will find it hard to prosecute senders from the mainland because they can't get hold of the source," he said. "The public should try to put pressure on WhatsApp to filter out promotional messages."
Privacy Commissioner for Personal Data Allan Chiang said with the widespread use of smartphones complaints about mobile phone advertising have become more common. But, he added, "a phone number alone is not considered personal data. It has to be matched to other personal information such as name and identity card number".
Chiang said the Office of the Privacy Commissioner for Personal Data will release its findings on the effects of promotions via mobile apps on privacy issues.
Carmen Yam, 16, a student at St Paul's Convent School, said she was annoyed by being forced to join WhatsApp groups that contained promotional materials. "I feel my privacy has been invaded as strangers could gain access to my pictures and personal details through the app. I only give my number to close friends," she said.
Adas Li, 17, of St Stephen's Girls' College, said she had received promotional messages on WhatsApp. "When I receive messages through the app, I have one big question: how did the companies get my number?" she said.