It's always a good idea to promote your show as widely as possible, and that includes social media of course. However, the problem with social media posts is that you are reliant on radio station managers trawling through a never ending list of posts, often with a very specific type of show as their target. The advantage of a listings site like Syndishows is that a radio station looking for a show of a certain genre can come and search through only the shows in that category. They can also find live shows, shows available via Dropbox, 2 hour shows etc.
By having lots of shows in one place, and a simple way of filtering, it makes life a lot easier for radio stations and gives you the best chance of getting noticed by the sort of broadcaster who is looking for your type of show.