I've been researching this a bit. I know bitcoinjs-lib has a standalone way of generating transactions and pushing them to arbitrary nodes. However, are sites polling third party api blockchains like blockr.io or are they all running nodes for each and every cryptocurrency they support (which isn't cheap really on a hosted machine) and polling them?
Inquiring minds want to know.
I think most (all?) serious operations are running their own bitcoin core node, which they can query (generally over the rpc interface).