I think the Q/A forums like this are great for lead generation because in a sense they are much like a buyer/agent interview process. Said client asks a question about a property or market or process of buying/selling and agent has an opportunity to answer and show their stuff/knoweldge. We have gotten quite a few clients off this forum because they have appreciated our honest tailored responses. Plus we have NOTHING to sell anyone therefore all of our answers are transparent and data driven which gets us great feedback.
I am unsure about advertising listings / ROI as we are an exclusive buyers agency with nothing to sell but my guess is the exposure to more buyers is positive (trulia ranks high on google).
People do tend to choose realtors based on friends / family suggestions and word of mouth. Buyer's should still beware and make sure they understand that - even if "referred" - in this business unfortunately not everyone's interests are aligned and 9 times out of 10 it is the buyer who ends up losing the most.
Interesting question and look forward to seeing other responses.