That's why Instagram, FaceBook, and Snapchat do so well- with those applications, you can take pictures directly in the app and post it, all in one moment.
But if you are photographing events for later posts, like I do as a food writer and blogger, I take a lot of photos to be used at a later date, or in another publication.
I always like to use my own photos- as I strive to be at every event or dinner I am writing about. It makes it easier for authentication- I was there, eating the sandwich I photographed, or listening the band I'm covering, or drinking the drink I ordered.
I'm in that moment as I want my readers to experience that moment as close as they can with me- and clear, concise, true photos attest to that.
On the flip side- if you were at an event or couldn't photograph it for whatever reason, do not just take another person's photo from social media.
That's unethical, incorrect, and false. It's best if you can find someone who was there and ask if they could share a photo in exchange for full credit with their name and social media handle.
Even if you are friends with someone, or co-workers using pictures from an event you both were at, it's important to ask permission.
If you didn't take it with your camera or smart phone- it's not yours.
If you need a photo - ask.
And protect yourself with apps like PicMonkey or Photomarkr to copyright your photos.