Bank transfer day
How much impact did it have? The reports I'm seeing around the net suggest that there was some, though it's early to tell. Smaller banks and credit unions, at least, have reported a major upswing in new accounts. The best report I've run across so far is here.
The biggest banks have total assets on the order of a trillion dollars, so the effect of people removing accounts containing just a few thousand will probably not be very damaging, unless really large numbers of people do it. For much smaller institutions, of course, the value of each individual customer is much greater. Frankly, I'd rather go somewhere where I'm more valued.
There has even been a meme circulating to the effect that giant banks would prefer to get rid of small depositors, on the grounds that such large numbers of small accounts are more trouble to administer than they're worth. Wells Fargo even responded to Bank Transfer Day by launching a new sub-bank called Abbot Downing, open only to customers with net worth above $50 million, as if to tell the 99% it doesn't care if they leave or not. Personally, I suspect psychological counter-warfare. Banks, from the biggest to the smallest, are always actively trying to recruit new customers, including customers of modest means; they wouldn't do that if they didn't see a net gain from having those customers. Note, too, how fast the big banks backed down on debit-card fees after complaints proliferated. That doesn't suggest that they're eager to lose our business, or even indifferent to it.
It will take a while to find out how much effect the bank transfer idea, as opposed to a single symbolic day, is having. Probably only a fraction of the people who feel motivated to move their money bothered to do it on that specific date. Myself, I'm still in the midst of switching; spreading the process over several days makes it easier and less glitch-prone than doing it all at once.
By the end of the year we should be able to get a clearer picture of the total impact. Obviously the big players aren't going to go out of business, but I suspect they'll at least feel enough of a pinch to realize that, in these days of greater class awareness, there's a price to be paid for angering the public.