I don't have the source (tried to find but buried), but Matt Cutts has stated because so many tech companies use .io as input/output webapp connotation, that behind the scenes, google treats it as a gTLD even though technically it's a ccTLD. I know because I did a ton of research for http://pineapple.io as well as talking to a lot of other people who were ranking their .io domain.
Likewise, there is a movement to get it officially branded as a gTLD just like .me, etc.
> The question came up about whether it matters which TLD (top level domain) you’re using. For example, do .com domains carry more weight than a .net, .us, .info, etc. He said that TLD doesn’t matter–that’s the way Larry and Sergey originally designed the Google algorithm. The algorithm doesn’t care where the page is located, it’s all about pagerank (LINKS) of the particular page. At the end of answering this question he did admit that they might have started to look at particularly cheap (and spammy) TLDs differently than other TLDs–or they might start considering TLD in their algorithm if they’re not already doing so.
Likewise, there is a movement to get it officially branded as a gTLD just like .me, etc.
I would say choosing a .io domain is just fine.