The true circumstances of the separation may never be fully disclosed, even to your friend. Even if they are known by your friend, it is possible that a confidentiality agreement as part of the separation may make it impossible to tell you.
Without speculation on the circumstances around the separation, organizations make these types of announcements when the employee in question had significant authority (ex. direct hiring/firing), legal agency (ex. able to negotiate and sign agreements on behalf of the organization or sue others on behalf of the organization), or the capacity to speak on behalf of the organization (i.e. authorized to talk to the press and make official statements and press releases).
This is a simple, mostly friendly way of letting others know that the person is no longer employed with the organization. Externally, this person is no longer authorized to make agreements on behalf of the organization or make speech/statements that are official on behalf of the organization. Internally it lets others know that this person should not have further access to organization resources or documents.
I'd expect the higher up the person was with the organization, the broader the communication would be, with some organizations making public press releases when key people with high level agency in the organization separate (or even before that when they announce their separation).
This prevents others from relying on a verbal agreement/signature with the person from assuming the agreement as being binding on the organization (and to let them know that they need to negotiate with someone else who has authority). Risk conscious organizations may even make these types of announcements for people with smaller signing authority (i.e. $5,000 or less).
As others have said, contacting via social media is a reasonable way to go, but depending on the circumstances they may not get back to you soon (may need a vacation, be looking for another position, or simply needs some separation from the previous organization and anyone tied with it).
If they do come back to the organization in some capacity, they may be a contractor, which would reinforce the need to make this type of announcement internally or externally to avoid confusion.