AudioGo admitted to being in dire financial straits last week when they shut down their UK operations. At that time they also announced that they were breaking up with Blackstone Audio, the US audiobook distributor acquired by AudioGo in late 2012.
It's not clear exactly what happened to the business relationship, but rumors had been circulating for a couple weeks (according to PW) that the companies might split. We do know that Blackstone is taking over AudioGo's US operations (and contracts?) as an independent company. This will bring Blackstone's catalog to around 9,000 titles.
Blackstone is currently directing all of AudioGo's US customers to www.downpour.com, Blackstone's own audiobook store, and they plan to eventually phase out the AudioGo branding over the next year or so.
AudioGo was formed in July 2010 when a group of private investors bought out BBC Audiobooks. The details for that deal were not disclosed, but the press release did note that AudioGo was "the leading audio publisher in the UK". They were already competing with Audible at that time, so you can take that for what you will.
Since it was a private company there hasn't been much in the way of news about growth, expansion, or even the state of the audiobook market.
What little public info I found indicates that the UK audiobook market generates around £126 million in revenue, and is expected to grow around 7% annually. That is rather anemic compared to the US market, which is growing by at least 12 to 14% a year.