Hey there,
I also have the same problem, I first had a family whom I ended up deleting to see if this will fix the problem, it seemed like it did, started a new family all over again and it seemed to have worked and I reached the 4th generation of this family and bam, it happened again to the family, I go out there and start convos and make friends but I lose all of them the same night. The dead ones stay though, even though the bar isn't even 1% full they just stay.
Ended up deleting this family as well, now, I'm playing with one sim both parents dead and yes, I played him through many lifetimes as a member mentioned above.
Here's what I noticed, this all happens when my sims join the culinary career, the first family I had one in the mixology branch and then it happened, same for the 2nd family, i had a sim join the chef career.
Now, my latsest sim reached the top of the culinary career and when i quit to join the secret agent career it happened all over again (level 5) it requires 5 friends and when I went out and made 7 friends, went back home and poof they were all gone, tried repairing the game but it didn't work.
Not sure if it's related to the culinary career or when a career needs friends to advance this starts happening, I just hope it gets fixed cause i'm not willing to replay all this again, especially not after I only have one plant left to complete the collection.
Anyway, Thanks a lot and sorry for writing too much, I just hope that the job thing helps you in any way possible.
Thanks again.