Should be easy to avoid with simply using another cookie jar or blocking third-party cookies. It seems the direct connection can only be made when a user is logged into a facebook or google profile. Otherwise the data can not be connected to a personal account without some degree of uncertainty and illegality.
G and F could use other data, like the browser fingerprint, OS information and the IP address to associate the data, which may be illegal, at least in Europe. Thus they probably use some other technique, for example creating pseudonymous shadow profiles and associating them based on similarity. In their front-end the data would just show clusters of profiles, which means they can claim they do not collect personal data, but from a quick glance it would be obvious to see connections between a user and "anonymous" clusters, if the similarity borders on 100%.
Thus a good practice would be to use a different Operating system and browser, together with the usual protective measures.
IP address is a very strong signal and short of Tor you cannot do much about that. I'm hesitant to recommend VPNs just due to the high level of trust you need to have in your provider. It isn't that easy to avoid tracking.
G and F could use other data, like the browser fingerprint, OS information and the IP address to associate the data, which may be illegal, at least in Europe. Thus they probably use some other technique, for example creating pseudonymous shadow profiles and associating them based on similarity. In their front-end the data would just show clusters of profiles, which means they can claim they do not collect personal data, but from a quick glance it would be obvious to see connections between a user and "anonymous" clusters, if the similarity borders on 100%.
Thus a good practice would be to use a different Operating system and browser, together with the usual protective measures.