GaijinPot has been receiving numerous reports from users about spam messages sent by senders impersonating GaijinPot using WhatsApp. These fake users promote non-existent job offers, trying to collect your personal and private information.
GaijinPot does not contact job seekers via WhatsApp or other SNS
Here is an example screenshot of the message you may have received:

GaijinPot asks our users to be cautious, as fraudsters may contact you with job offers to get your personal information, request payments, etc. GaijinPot and its parent company, GPlusMedia Inc., are not liable for any damage caused by users replying to fake message requests.
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I received 2 more recruiters which I assumed from foreign countries but I reported them as spam and blocked them.
On top of it all, I connected with unknown guys twice and their resumes disappeared as soon as they got my number on WhatsApp or LINE… I am afraid that using our real names along with mobile numbers is becoming dangerous even though I keep LI only site. I think LI was better before…
Never submit a copy of your residency card online to someone. It’s against the law in Japan for someone to ask you for your residence card online without a real job contract. Only after a face to face meeting at an office where a company is located should you provide your residence card for reference to the potential employer! They can make a copy of you sign a job contract with them but if there is no job contract then they do not have the legal power to make a copy of your residence card!