Gregg:
You're confusing FREE with PD. Morguefile absolutely and positively requires that you post attribution for the creator.
http://www.morguefile.com/about . You can use Pixababy images on social platforms, according to their terms, but I'd be careful. RGB Stock
sometimes requires that you contact the image owner for permission. See this:
http://www.rgbstock.com/faq and look down, to the licensing information. Even RGBstock doesn't warrant and represent that they are 100% sure that EVERY image is usable without permission.
Really, Gregg: just use something like Hootsuite, or do the dreaded homework. There's no such thing as a free lunch, essentially, in the photographic, digital-age world. I mean, I use CC searches all the time, and find adequate images for articles, blog posts, etc. If you're posting about something from TV, say, look for media kits. Ditto for logos. But it's just a lot of work, essentially, to use images without having to do the trackdown sweat and obtain the necessary licenses, releases, yadda.
Hitch