I'm the Director of Platform at 500px. We're not planning on leaving Rails any time soon, but we are slowly moving parts of the monolith into new, separate microservices, some of which will be in Go, but others may be in Sinatra or Rails -- we'll choose the right technology for the service in each case.
Speed. Our resizer service needs to respond within about 200 milliseconds with an image, and our users tend to upload very large photos (sometimes hundreds of megabytes in size). ImageMagick/GraphicsMagick unfortunately are just not performant enough to meet this goal for these larges images.
Of course, VIPS has its own drawbacks -- it took us quite a while to determine the right configuration of filters and resize operations to produce a good quality image, something that comes pretty much built-in with IM or GM -- but its performance made the investment worthwhile.
I can't speak to performance, as we never actually ran Kafka in production, but we chose to use Kinesis because we didn't want to manage our own Kafka (& thus Zookeeper) cluster, at least for now. 500px is run by a fairly small group of engineers, so we try to use cloud services, like AWS Kinesis, when it makes sense from a cost vs time investment perspective. Of course, at some future point we may reevaluate this decision and move onto an internally-managed solution such as Kafka.