My impression is this: in a small float stock -- FULL, for example -- a well-timed message board post can make a difference.
In a large float stock like IBM, not so much.
Even with PSEC, I don't believe a significant post -- pump or dump -- will do much.
I do believe heavy share investors read this board and groups of them can move in reaction to a post, but I still don't believe it is enough volume to make a dent.
Here are some consequences of posting I can imagine being realistic:
individual investors make changes -- so you might influence folks on this message board to sell or buy, but not enough to impact share price
enough OT posting can drive away meaningful conversation about a stock or the company
There are at best twenty people who frequent any given board, unless it is a large-cap and even then I doubt it's more then fifty.
No matter what company it is, discussions on a message board would have zero impact on the stock price.
What HAS become more influential is a blog post on something like Seeking Alpha, where any idiot can post an opinion on a stock and it may move the market. The reason is that SA and Motley Fool (which has a lesser impact because of the fomulaic nature of most of the posters), is picked up by other financial boards (finviz, aol, and yahoo!) as a "news headline" and people are driven by the exposure.
I know I base part of my choices on what I read on yahoo MB's.The best MB for good investing ideas is AGNC hands down,I read it every day and it's not just pumping AGNC they have a lot of other good OT trading ideas.It's hard to come up with ideas on your own and all paid investment services that I tried were a waste of time and money.I don't blindly follow the board pied piper,I learned my lesson the hard way,just pick and choose ideas and do home work on my own before jumping into anything.