I feel it's source is from some social platform like that or something like a calender app that links all these together e-mail, meetings with contact details etc. Even having separate email usernames only narrows it down so far.
Don't think it's right to speculate TBH you can cause an unnecessary panic, people that are around these breaches have been trying to find the source for several months now, it's taken that long to add it all on to HIBP.
The one piece of information to take away from these breaches is don't re-use passwords.