Greetings Lewis
So yes URL shortners can and will get indexed if there are enough links to them, there are 71.8 million bit.ly links currently indexed in Google... I did a check of my travel site and noticed this Buffer UTM link in GWT Top Pages report http://travel-network.co/review/aggelika?utm_content=bufferebb76&utm_medium=social&utm_source=plus.google.com&utm_campaign=TravelNetwork.net.au it was indexed and getting impressions but it was only a small amount last month and I checked and now only the canonical version remains indexed.
But it was only the Google+ version being indexed as Buffer creates a unique URL for each share on a social network, so the only question is did you manually share the Buffer link on Facebook & Twitter so it was actually the same link? Also did you check to confirm your page has the correct canonical tag in place so it reduces the chances Google will get confused and indexed the Buffer link instead of the actual blog post link.
But I couldn't find any other examples of Buffer links being indexed, if you are worried about the situation would could use the Buffer settings and select "No Shortnening" option.
Hope that helps a little.
David