The free version of Canva is great for creating social media graphics: https://www.canva.com/
Use our Facebook login to get into our Canva account.
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Adobe Sparks is also used for creating social media graphics (known as posts) but it's a little more robust than Canva because it allows you to create video and what they call "visual stories."
https://spark.adobe.com/make/social-media-graphics/