Before hiring someone, most startups try to find out as much as possible about them by hitting Google and checking their publicly available social media profiles (Twitter, Facebook, Instagram, etc).
To what extend should their behavior on social media effect your hiring decision? For an example, if lets say the candidate with the perfect in-person interview has an obscene joke posted on their Facebook feed. Is that an indication that they shouldn't be hired, or just that they have a different type of sense of humor.
When going over someone's social media, I'd use caution in passing judgement too quickly, especially against them. Usually, with social media, their social media is personal, and you still want to allow their personal life to be theirs, and to be able to still have it outside of work with you.
What would be a legitimate red flag is if they talk about blowing off work, or to a lesser degree, things like frequently complaining about a boss, coworkers, or their work. (Few people have 0 complaints about their boss, and social media is a popular place to vent about things like that among friends. So don't read too much into it, unless it's a pattern.) Most people understand that work-appropriate behavior and play-appropriate behavior aren't the same thing, and be better behaved while at work.
On the other hand, if their personal social media accounts are full of insightful work-related comments, it is pretty safe to interpret that as coming from somebody who is really passionate about what they're doing. It doesn't mean they're necessarily good at it, but passion is a really nice thing to have.
I guess I'm saying that I'd be generous about allowing social media to be a point in their favor (depending on the content) and stingy about giving a point against them for it.
Definitely, a single obscene joke in a three year Facebook history wouldn't be a deal breaker for me.