I remember when flashtweeet.com was first made (by a guy on here) and it was free to use, can't have been any more than 3 months ago. I had never used twitter and started 2 accounts. I used the mass adding software to follow roughly 1000 on each account then sent out 1 tweet on each account. The result was like 600 odd on one account and 700 odd on the other following back. Also a shit load of clicks from the tweet and to date i make a lead every now and then still.
Fast forward to now, flashtweet costs like $6 per month or something like that (which is nothing) but I've been using a similar free service. I follow the same amount of people and end up having like 50-100 following back. And the actual amount of these who click the links in the tweets are basically none.
This is in the space of only 3 months and is basically due to the mass amount of us spamming twitter. I just wish I had went ape shit early doors with twitter and really raped it as I only realise now how much money could be made. I agree with the guys who have posted already on this thread, there will definitely be money in the niches that the vast majority of marketers don't think to target. Most people can't see past MMO and diet offers.