Hmm, when thinking of this particular venue/format (forums in general) as a platform it is important to recognize that it is something like an omnidirectional microphone
You don't "have the mic." except that you do...but so does she and he and they. Imagine a church service something like a game show with pre-recorded messages that we each have our own private studio for. The game show element comes in with whomever hits the "send" button first will be seen first but not necessarily "heard" first (if at all). Except this isn't a "service" although I do certainly believe the Lord operates through this venue.
It's more of a watercooler or maybe something like a booth (and I appreciate the image of an assortment of soapboxers). Some are selling, some are buying, some are observing, and some are milling about. I'm sure there are a myriad of different reasons people congregate in these venues.
So "to preach" AT someone this particular venue would be unsuited for. Or at least I am starting to find that to be so, except for perhaps the blog section (which in this forum gets hardly any traffic).
To simply "preach" I suppose it could be likened unto street preaching. Listen or don't. So it appears to be acceptable here, but since it is a confined place it makes sense that you should operate with the understanding that many may listen but each have different reasons for listening. Sometimes it may be to simply inspect that you should indeed have a place to speak and these would "speak out" if they felt you did not.
I think the word is what gives us authority. Most here are image-bearers...which demands some sharpening. Rebuke, chastisement, correction, commiseration, encouragement, and edification.
However, we are not THE word so our authority has limits on how we carry each of these out and stewardship and accountability do apply. Particularly to not go beyond what is written.
I wasn't intending to be overlong about this, but yes I think we can. It is hard to consider the audience when your audience is potentially incredibly wide, and so I overlook a lot personally. As well as factoring in that not everyone understands their own role within the body and how to be in accord.
I've only felt like I was "preached at" once here, and I still felt it wise to keep it to myself and consider where they were coming from. I may have overstepped, I'm not sure still. Perhaps misplaced chastisement, but perhaps not. I've overstepped before by speaking about things that shouldn't have been spoken about adding a strong disclaimer and then somehow only awakened a person's curiosity toward things they might not have ever come across, so there is that.
We most certainly do have authority but since it is so vague in this format I think it best to be diplomatic unless directed otherwise.