If you bitch, whine and attack on social media that’s who you are to the rest of the world. It’s doesn’t matter if you’re only venting when you have a bad day. If all people see is your bad day reactions they’ll assume that’s who you are. And they’re probably correct. That doesn’t mean you shouldn’t post challenging ideas or engage in real debate online. It just means you should probably avoid slander, witch hunts and being an asshole. It’s a safe assumption that the person going on the attack and judging others online is probably that same person offline.
The reverse is also true. If you promote creative ideas, interesting dialog and support via social media that’s who you are to your readers.
Applying for a job? You can expect a hiring manager to look at your Facebook page. Probably before they find you on LinkedIn. If you’ve called sick into work your manager would be silly not to check your updates. Think before you post. What does your Facebook or Instagram page say about you as a potential employee? A collaborator? A friend?