The simple rule is: If you don't own it, don't use it.
Most social media sites offer copyright protection to the uploader. If you load it, it is yours. Unless the uploader specifically states that other people can use it, it is theirs and theirs alone.
There are exceptions, some of them strange. Facebook TOS says that they also own what you post. So if you post a picture of you, they get away with letting other people share your post with the rest of Facebook. Even under these conditions though, taking someone else's work there and posting it here, could get you into trouble.
If you really need an image that you cannot make, there are ways to know what you can use. Without getting too complex:
What you want to look for is the copyright restriction of the image. It will tell you the terms that are okay. If there isn't anything listed, don't use it.
If the copyright info says "Public Domain", you can use it without restriction.
If it's "Creative Commons", there will be information listed on how the owner is okay with you using it. Some allow you to make money off of it, some don't.
If it's "Royalty Free", most places are selling the image, but there is no charge per use. Just a charge upon purchase.
In the end, do a little digging, and if you don't know, don't risk it.