The reproduction or publication of any company's intellectual property, which includes logos, should only be done with the permission of the company(s) involved. If a company owns a logo which is registered then it is against the law in some countries (including the UK) to reproduce it anywhere without seeking consent. If the logo is trademarked, then it is not illegal (in the UK) to use it, but you should ask to use it and they may want to see how you will display it before allowing you to publish. Any other logo you use, if in doubt, ask first.
As a graphic designer employed by a large wholesaler I have to deal with hundreds of company logos, some registered, some trademarked and I regularly receive information and agreements on how the logos and product images are to be reproduced for print or screen. I think a large web company like Linkedin will have a prior agreement with most companies to use their logos throughout the site.