Well, I've been involved with NaNoWriMo (National Novel Writing Month - in November) for the past two or three years, and I've worked on three books. One I have finished (but it will never see the light of day, haha), one I am continually and slowly working on, and another I am actively working on at the moment. I do think that a lot of people don't seem to finish the things they start, but often they come back to them.
Personally, I procrastinate a lot and take extremely long hiatuses, and I don't think I would ever be able to be a professional fiction writer unless I really knuckled down. However, I still have a passion for writing and would love to finish my current book.. I'll see where that goes.
I have used Yahoo Answers to get feedback on another idea I have for a novel, and I do feel like I received some very valuable advice from it. I actually do try to give people very direct, honest advice about their writing on here, and I don't know if they are committed enough to their stories to finish them.
However, that is their own personal business, so I honestly don't mind. When I see people who I can tell have fallen in love with their stories, I do my absolute best to help them in any way I can, and I leave it at that.