There are a ton of resources out there for social media content creation, and I'm sure you even have different ones that I'd love to hear about! I mostly use Photoshop for all things content, but here are a few apps/sites that I use or have used that I really like.
Some notes before I get into it:
~Just because an app does something trendy, doesn't mean you need to do it. Make sure that whatever you create is ON BRAND. Your brand equity is at stake so take the time to create content that is visually appealing but looks like your brand.
~Nothing and I mean nothing can replace a graphic designer (and this coming from a self-proclaimed design hack.) To really stay on brand, hire a graphic designer to create a variety of templates based on your brand assets, then you can work within those to create content till the cows come home. This ensures that everything that goes out into the world has your logo, has your look & feel and has your personality.
1. PicLab (upgraded version) app: Easily drop text on an image, change some minor settings like contrast, exposure, blur, add some "stickers" (cute design elements) and add a filter.
2. Canva: Canva has always been pretty cool, but they have upped their game recently with the launch of infographic templates and (soon) Canva for work. This site has templates for each platform, at the optimized sizes. You can upload your own images & logos, then use their design app to make them look good. My recommendation on Canva is to try to customize the art TO YOUR BRAND. Yes, Lobster font with a neato layout with curlycues is great and has mass appeal, but is it true to your brand?
3. SquareReady app: Upload a photo to keep your image's dimensions in a square layout. Perfect for the times where you want your landscape-oriented image (the full image) Instagram ready.
4. Photoshop Express App: I would be remiss if I didn't mention photography apps! I use Photoshop Express to do my on-the-fly photo editing for social media content. This app is perfect for making your images pop with all the standard PS features. If you're comfortable with Photoshop at all, you'll love this one.
5. Manual App: Take a bit more control of your phone's camera functions. This app allows you to change the ISO (perfect for low light conditions) and shutter speed. The best thing about this app - changing the white balance. (My post on white balance.) Understanding the light you're in changes your photography/visual content game. Trust me.