I read the original post as being successful from the view of the companies delivering the airdrops, not the people receiving them...but I may be wrong. If it was about the companies distributing the airdrops, then I think it varies massively depending on the project. Basically it comes down to the demographic that they want to promote to. It is easy to push a project and give away a few dollars worth of tokens to thousands of people, but getting actual investors to part with their money after seeing the airdrop promoting is a lot tougher.
Conversion rates are likely to be tiny, especially if the airdrop has not involved any real effort to take part, as it will fill up with thousands of people who never invest any money and will only ever go for freebies. In my opinion, the ones that give the best returns for the ICO are ones that involve a bit of effort, and therefore filter out many of the freebie chasers. Not knocking anyone that wants freebies, we all do, but companies need real money invested otherwise nobody makes any money.
Most of the airdrops (as someone has mentioned above) end up being worth next to nothing, and this is a big clue that those airdrop campaigns did not work. I will always keep an eye out for decent looking airdrops, but have stopped signing up for most of them. It just doesn't benefit me or the company giving it away, so not going to waste my time. With the thousands of ICOs which will take place before the end of the year, it is a tough market to grab attention, especially for smaller projects. I think the marketing departments need to up their game and be more original, especially with the advertising restrictions on certain social media channels now.
Anyway, here's to another bull run so we can all make some decent money over the next year...