I'm having trouble setting up a proper/sharp looking social media post designs for facebook. I have 5 or 6 different designs, all are the same size, I export them all the same way (Save for web -> PNG24), but for some reason some of them show up in a very poor quality when posted on facebook. They are all in a good resolution too. Is there some specific things I'm not paying attention to, like the final file size (KB's), colors etc.?
Any particular reason to export PNG's? Try exporting to JPG, which in many cases can look better than a PNG. Also see this: https://www.facebook.com/help/266520536764594
Hi I can offer you my routine as an aid, cannot get into your set up deeply unfortunately due to time. For reference I am on a Mac Pro 5,1 BTO with 32GB Ram, 12 Core 3GHz, OS 10.12.6, Photoshop CS6 - whilst this may appear old to some, its a rock solid combination and capable of all I need.
I build in Photoshop and start all art at 300 dpi for first photo correction and retouching so I have a good master for images. I build vector icons and logos in Illustrator where possible for the same reason... unless I am on short time - the click icon in my sample post for example. For social media I tend to build the banners at deployment size so I can judge the quality at edit, particularly text legibility. Whilst different channels do prefer different sizes and will crop deep vertical images 1000 pixels x 1000 pixels @ 72 dpi and RGB typically works fine on FB and Twitter.
I then save out to JPG using Photoshop's built in web optimiser under Save For Web - logical because it gives you quality control v file size and you don't want to be posting anything too large to SM or (1) it will be compressed by the channels gatekeeper (during upload) and (2) mobile devices will struggle to download it - I went for 'Quality = 90' on these as it is a premium client but you can go down to 60 and still get good results.
On a Mac you can quickly check the integrity by opening the file in Preview but dont make any changes in this app or it will over save the Photoshop output (and typically go up in size).
And here is the actual image file if you want to use it as a template: https://pbs.twimg.com/media/D-tE30aWsAE7VKk.jpg
Note: on a commercial banner like this I put the offer detail in the post text, typically with a website link for the campaign landing page. The call to action (Join Now) is a psychological hook. You don't want too much message on the images as people wont read it. The image is key clearly - in this case I am appealing to an off peak audience, + 25, female, mum through to grans and the group / community interaction is deliberate for this audience... they might be at the gym for company or simply to get out of the house / daily routine as much as fitness.
I am sure others may have a different approach / opinion although this works for me and in short time if needs be - 25 mins if I am into the client, have media on board and know the target well. Hope this helps.