Because facebook only has my pictures if I choose to post them there. This is easy to avoid. Facebook only has my browsing habits if I choose to allow content from their servers while viewing non-facebook content. This is also avoidable, albeit less easily. Avoiding sending email to gmail users is far more difficult. Avoiding receiving email from gmail users is even more difficult. Additionally, the cost (at least measured subjectively in terms of inconvenience) of avoiding all contact with gmail users is far greater than the cost of avoiding facebook.

So, to answer your question more directly, facebook doesn't "get a pass". Facebook simply doesn't get used.