According to the court, website owners "must provide, at the time of their collection, certain information to those visitors such as, for example, its identity and the purposes of the [data] processing". By extension, the ECJ's decision also applies to services like Twitter and LinkedIn.

Indeed the "by extension" bit is also important. It was why I removed Twitter's "follow" embed from my own blog and replaced it with my own custom code: so that they couldn't collect data on my users.