Even with a solid resume and cover letter, your activity online could ruin your chances of landing a job. More employers are headed to social media sites in order to get a closer look at their applicants.

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Using LinkedIn, hiring managers could corroborate the work history you’ve shared. But the bigger threat is what they find on sites such as Twitter and Facebook. How do you portray yourself? What do you value? Do you have too much fun when you’re away from the office?

“Even if they’re not connected to you, they may have a connection to you through somebody else,” said Joy Schneer, professor of management at Rider University. “So you’re never really sure who’s reading what you’re posting.”

Schneer said many job seekers, particularly among the millennial generation, forget that the world is watching when a tweet or photo is posted.

But social media doesn’t only have to hurt you. It can actually help you stand out from the rest of the applicant pool.

“If you regularly tweet and you have an amazing following and you’re posting information on Twitter that’s relevant to your field….that’s going to be seen by an employer that you’re very professional and you’re devoted to your profession and you’re sharing your knowledge with others,” she said.

An online presence, though, is essentially a must these days, whether you like it or not. Schneer said an employer may find it odd if an applicant has no profiles anywhere. That could hint at a shady past that’s been erased from the Web.

“It’s also, I think, seen that you’re not keeping up with technology, that you’re not engaged in social media, and social media is becoming such a big part of how business is done,” Schneer said.