I have lead records into which I had previously merged new lead records that were created when they had a social share via LinkedIn, so these lead records have the unique LinkedIn Social Profile URL.
At a later date some of these same people used our Social Sharing Button to share content on LinkedIn likely from a different macine/browser, so a new lead record was created. The IP address and the unique LinkedIn Social Profile URL are the same on the existing and new records.
I know that Marketo uses the email address to ensure that a duplicate lead record isn't created especially when the cookie is not present, but with the Social App, the email address is frequently not captured, but it does have the unique Social Profile URLs that could serve the same function.
Please implement more ways to tie Social Share activity to existing lead records, rather than create a new lead record.