i'd say, twitter can be useful for some, who uses different approach, but i have similar findings
my best personal experience with twitter moneywise, when twtmob paid $32 for a single tweet and then some on one of my 30k accs lol, this wasn't yesterday though
on a side note, a huge percentage of the twitter traffic comes from every kind of bots and i think bit.ly doesn't filter those:
http://blog.manageflitter.com/bots-what-why-and-their-impact-your-twitter-click-through-statistics
bot clicks can be as high as 3/4 or even more of the total clicks, but even then it's a pretty shitty conversion rate you got there