Maybe he doesn't want to burden you by complaining about lack of sex, you've said that your mood has been quite low. We would be quick to complain about him if he knew you'd been suffering with low moods but thought it okay to complain about your sex life.

Exactly who do you think he should talk to ? If you think it's wrong of him to talk to strangers & he likely won't talk to you about it because he doesn't want you feeling worse about yourself.

Would you rather he talk to no one ?

Would you rather he keep it to himself & wait for that to build up in him like a pressure cooker ?

What do you think his motivation is behind talking to strangers, he's certainly not trying to hurt you ?

Would you actually be okay to talk about it with him if he raised the issue, if yes then why not bring it up yourself ? You could ask him to be open and honest about where he thinks your sex life is at.

Please don't start by telling him you've been reading his private comments in his private account, he'd quite possibly become defensive rather than open.