I doubt the le agency has the budget to devote tracking and usage to compile evidence against hookers in court. But if they did then that's what they're doing. Not merely for just visiting adult websites.
Originally Posted by Out_of_Bounds
Funding comes from the broad net cast for trafficking and underage victims. The personnel is drawn from the local agencies with FBi/DHS oversight and nationally pooled assets. You will find that most of the computer gurus are local LE and not Federal agents, and most of the testimony comes from the locals. In addition the Feds use contract labor from nonLE gurus for developing the evidence.
If, and I am saying "If," the hooker is visiting the site to check her hits or PM's in response to her ad she has posted on the site, then the link to the ad is made with the source, and a soliciting case can be made against the hooker. The hooker only has to "offer" not fuck.
That is probably well beyond the scope of the initial question, but it certain addresses the dismissive "tin hat" bullshit.
And I agree with you: LE also gathers evidence from social websites.