I realize you're not engaging leftist theory seriously here, but if you were I would recommend this paper on the topic of digital new media as viewed through a Marxist and political economy framework.

Regardless, I don't see the exploitation of user activity as a theft of 'personal property'(nor would marx), it is closer to the private ownership of common resources (i.e. private ownership of land and the resources on it, land being the platform where free human activity occurs, and the raw resource as the data being collected). A leftist might assert user activity and communication as a communally shared resource, not one privately exploited, and the resulting tools that utilize that common resource as one that is collectively shared, not privately owned.

Once again, it's not about theft