Question: Is Sony monitoring PSN? Answer: Yes. Yes they are.
An updated version of Sony's Software Usage Terms for the PS4 stipulate that Sony may "monitor and record" communications between users made through PSN. The same section of the terms stipulates that Sony also reserves the right to remove any content from users' accounts without notice at their discretion. These rules apply to "voice and text communications," as well as streaming video made with the PS4 "share" button.
These powers give Sony the ability to "comply with the law," enforce their terms of service and "protect the personal safety of our employees and users," among other things.
"Any information collected in this way, for example, your UGM, the content of your voice and text communications, video of your gameplay, the time and location of your activities, and your name, your PSN Online ID and IP address, may be used by us or our affiliated companies to enforce these Terms and the SEN Terms of Service, to comply with the law, to protect our rights and those of our licensors and users, and to protect the personal safety of our employees and users. This information may be passed to the police or other appropriate authorities. By accepting these Software Usage Terms, you expressly consent to this."
Sony says it cannot monitor "all PSN activity," implying that, just because can listen in, doesn't mean they are every time you load your machine.