At ZIB there is a mailing list alumnizib.de, in which former ZIB employees are registered.
On the one hand, the purpose of this list is to give ZIB staff the opportunity to get into contact with former colleagues, e.g. in order to invite them to staff excursions or Christmas parties. On the other hand, it makes it easier for former colleagues to get and stay in contact with each other.
If you are a former ZIB colleague, you can register for the mailing list here.