The developers of Helldivers at Arrowhead seem keenly aware of the issues that players are facing now that the game is released on the PS4 and PS Vita. As noted in a previous post, they will be fixing the bug where player progression is wiped when the controller is turned off.
Beyond that, they have issued a FAQ addressing all of the various issues players are having with the game, including the inability to find friends' games on the playlist.
Here's a description of the problem and what they plan to do about it.
The issue: This is a very frustrating issue for many as Helldivers is primarily a co-op game and you really want to play with your friends. This is more common when trying to find friends on other platforms than your own, such as between PS4 and PS3. It comes from the fact that we are pioneering cross-play functionality and it is very much a learning experience.
What we are doing about it: We are looking at improving the discovery for friends, especially cross-platform. This is unfortunately not an easy fix and will take a while for us to sort out. I’m very sorry for the inconvenience and we thank you for your patience.
What you can do about it: Setting your matchmaking filters to closely match what your friend is currently playing seems to increase the likelihood of them showing up. You can also both try to find the same public game and both join it.
I tried setting my matchmaking filters to sync up with that of my friends' and it worked, so there you go.
