Is it legel to use a company logo on our job networking site? These are large companies that use our site to post job openings.

It is a risky proposition to use company names, brands, and/or logos on your website or in another marketing materials without permission from the company/brand owner to do so.  From your post, it is little unclear to me exactly what your business is and how you are using third party company logos.  I would need to learn more from you about the specifics of your situation in order to provide more specific feedback.  However, as a general principal, use of a third party's name, brand, and/or logos in a commercial manner, without their permission, particularly if your use suggests a relationship, affiliation, sponsorship or some type of connection between the third party and your business is potentially actionable and could expose you to legal liability, injunctive relief (such as a temporary restraining order), and/or monetary damages.