Yes, there are legal issues.
And no, Facebook etc. dont do it, their users do it. Its just too much trouble (and bad PR) to get the legal club on them in 99.99% of cases.
Of course as a platform you may be responsible but that depends on the country you are in.
If I paste a link into a FB post it fetches some details that include an image from the post and part of the text. That at least gives people an idea of what they will get if they click it.
Thats an exclusion to copyright.
Alas, the idiots here in Germany e.g. have passed a law (and try to do it on EU scale) where this is illegal.
Dont know what FB does here, I dont use it because I like my rights.
When you make a website you place some meta-information on the page that can be shared using the Open Graph protocol. I do not believe that there is anything wrong with including this on the site.
And it is Facebook that include this in their html. I am not sure about the legality, but the images has been put on the linked site for this exact purpose. I just did it myself on the comic web-site I created recently.
You can see it here. Look for the meta property- and the meta name-tag.
There is no qestion about that there is no problem with things that are allowed. But on 99% of websites, taking content is not allowed.
But just making the meta-tag shows that this is for linking use - I think it must be legal.
With OpenGraph tags you actually tell the site which wants to share your info: "Here is my data, please take it and share it."
Legal issues aside, of course :)