@Gwenhwyfar For me, it is designing an invitation or buying it, or creating an 'event' on FB (which I've been invited to a few parties via).
When I had my last 'big Birthday' I drew up a postcard sized invitation that I would have printed out for the previous one and sent it via social media to most people I invited to my party. However I also had a few other do's (went for lunch with the girls I used to go to school with / went for an afternoon tea with three close friends / went out for a meal with friends from work / went fr a meal with dh's extended family) and didn't send them a "drawn up invitation", jst phoned or WhatsApped etc.
The party I 'hosted' (paid for the hall / entertainment / food), the other things we all paid for our own.
So, although it wasn't my contribution to this thread - I was quoting another poster - , it struck me that , yes, that fits into what I would do myself, and also how I would "read" another invitation.
Same as when my dd turned 18 a few months ago - she made an invitation and sent it (on social media) to people she was inviting to her party (which we paid for the room and the food), but when she also arranged to go out for cocktails with one group and meet for a meal with another group, and everyone was paying for themselves, there was no formal invitation, just normal social media to arrange to meet up like any other social arrangement they make.
So don't think it is a generational thing, as the same has been true for the invitations she's received this year. Where is has been a "hosted" (ie paid for) party, she's had an invitation and where friends are paying for themselves, they've just arranged it without an invitation.