With very few exceptions, all digital broadcasting is done on commercial (non government)stations here in the states. It is done voluntarily by the stations. There is no current requirement that they have any digital broadcast. Many of the newer receivers are able to pick up the digital broadcast, but there is no requirement that all receivers be equipped with digital receivers. I have a digital receiver and sound is great (CD quality) but reception is not as wide spread as the analog signal. The receiver will automatically switch to analog at the loss of the digital signal. On a side note, at one of the transmitter sites here, 7 FM stations share 1 antenna for there analog broadcast. A very large combiner connects all 7 transmitters to the single antenna.
In Ireland we have 2 public service broadcast FM stations : RTE Radio 1 and RnaG (in Gaelic only). RTE also operates 2fm (music driven station), and LyricFm (Classical/arts) on a purely commercial basis. On DAB they've been playing around with a few digital channels non-commercially.
We then have Today fm and Newstalk which are national FM commercial operators.
Then there are a whole range of regional stations.
Then, the section with the largest audience share, the local commercial stations which typically target one city or county. Larger population centres have quite a few of these, where as in rural areas there might just be one local station.
Finally, there's a whole layer of community stations which are operated by community groups, universities, hospitals etc. These are often manned mostly by volunteers and typically have a low powered FM transmitter covering a fairly small area.
You also get small stations going on air for special events etc.
The radio sector here is very heavily dominated by commercial radio in much the same way as the US.
Commercial AM licenses have been available for decades, but no stations seem very interested in them anymore.
With regards to site sharing, again it would just come down to practicalities and what stations agree upon. ComReg, our equivalent of the FCC encourages broadcasters and telecommunications companies to share masts (towers) where possible to minimise visual impact and make it easier to monitor RF emissions.
