If Open Graph tags are present, LinkedIn's crawler will not have to
rely on it's own analysis to determine what content will be shared,
which improves the likelihood that the information that is shared is
exactly what you intended.
It tells me that even if Open Graph information is not attached, LinkedIn can pull this data based on its own analysis. And in case of YouTube it seems to be the case, as I couldn't find any Open Graph tags added to YouTube pages.
LinkedIn revised their site recently, so there are a ton of old links just redirecting to the developer support homepage. Here is an updated link to the relevant page on LinkedIn's support site (as of Feb 16, 2015): https://developer.linkedin.com/docs/share-on-linkedin
Its best to use customize url approach. And its the easiest. Found this one. It will open a popup window and you dont need any bs authentication issues because of w_share and all.
LinkedIn has updated their api and the sharing url's no longer works. Now you can only use the url query parameter. Any other parameter is going to be removed from the url by LinkedIn.
Now you're forced to use oAuth and interact with the linkedin API to share content on behalf of a user.
Once upon a time, you could use these params: title, summary, source. But if you look closely at all of the documentation, there is actually still a way to still set summary, title, etc.! Put these in the <head> block of the page you want to share...
<meta property='og:title' content='Title of the article"/>
Not sure you did everything right? Take the URL of the page you are sharing (i.e., example.com, not linkedin.com/share?url=example.com), and input that URL into the following: LinkedIn Post Inspector. This will tell you everything about how your URL is being shared!
This also pulls/invalidates the current cache of your page, and then refreshes it (in case you have a stuck, cached version of your page in LinkedIn's database). Because it pulls the cache, then refreshes it, sometimes it's best to use the LinkedIn Post Inspector twice, and use the second result as the expected output.
Still not sure? Here's an online demo I built with 20+ social share services. Inspect the source code and find out for yourself how exactly the LinkedIn sharing is working.
Step 4 - Finding More Social Sharing Services and Their Share URLs
I have been maintaining a Github Repo that's been tracking social-share URL formats since 2012, check it out: Github: Social Share URLs.