The question is, how has 4 billion impressions converted to sales? The answer, not well.
I obviously haven't sold 4 billion books which would be 100% conversion.
I don't expect to. Typical click conversion rates online are <1%.
1% of 4,000,000,000,00 is 40 million.
I haven't sold 40 million eBooks either.
0.01% of 4billion is 400,000.
I still haven't sold 400,000.
Even if we accord every sale of mine to twitter, we're talking about a conversion rate of around 0.0025%, insanely low.
And, let's be honest, most of my sales were down to Amazon. Hitting the top 100 in Mysteries & Thrillers generated many thousands of my downloads.
To try and make an educated guess at how much twitter accounts for, I've looked at 2 things:
1. The click rate on the links I post on twitter. I use goo.gl shortlinks so I can track # clicks. The average link had less than 50 views, with a few notable exceptions (and all of these exceptions were for tweets about free eBooks which were clicked 809 times each on average over the last year). I have no way of telling how that then converted from click to blurb read, to sample, or to purchase. Anecdotally, I see little to no sales spike when I tweet.
2. The number of tweets on any given day versus the number of sales on that day. I've recorded all of this - which is a pain as Amazon give cumulative totals so it means looking every day. I wasn't exact in recording all this (e.g. one day I'd note my sales at 4pm, another at 9pm etc) but in general there was little to no positive correlation between tweets and sales.
The one thing I have seen a correlation between is # new follows in a week and # sales but it's not a significant chunk of my sales. That suggests either a) your window of opportunity to sell is limited, or b) people are following because they bought rather than buying because they follow.
All in all, I'd say twitter is a net loss when I consider how many sales I think I've made, and how much time it has taken interacting on twitter to get those sales. Of course, twitter isn't all about selling, it's about connecting. There's a lot of fun to be had, and some long term recognition/ brand building to be done... but there has to be a ROI on any business activity. In my experience, despite huge numbers of impressions generated, twitter pays dividends only in social terms. I'll still use it, but for fun rather than for profit.