I'm dealing with this issue right now with Snip.it. They were clearly in talks with Yahoo! about being purchased (and potentially others as they were putting out surveys every few months about usage and purpose that I filled out each and every time), but didn't inform any of us that the site would literally be shut off late one day and we would be unable to retrieve (or even look at in read-only mode) all of the data we'd put on the site.
We were initially able to grab a PDF or HTML file of the links we'd posted, but none of the additional descriptions we'd place along with them. They told us to try Kippt and Diigo but the way they'd formatted the file wasn't friendly to either of these services, which would lump them all together instead of separate them by the categories we'd placed them in (although it would "tag" them with the category, but that doesn't help me navigate when my tags aren't listed out anywhere). When I complained about the descriptions issue on their Facebook page, they added them a few days later and another service (keeeb) tried to reach out and ask them how they could better serve the fallout (not sure if they responded, but keeeb looks like a neat service). I got in touch with Kippt to ask if they could figure out a way to get the imports to work correctly since they were being advertised, and they asked for my export file and said they'd work on it, but I haven't heard anything back.
I don't care if your service is free or not (and I even informed Snip.it I would have paid for the service, as some people replied on the linked post did as well), you owe it to the users that gave a shit about you and wanted to see you succeed to have some sort of grace period, and better, to escort them to similar services so they aren't left completely in the dust, having spent months and years of time contributing to and advertising for your company. At least in my situation, they tried to do the latter, even though these services weren't actually anything like their own and couldn't parse the data.
Whatever Yahoo! plans on doing with the Snip.it team, I won't be using it. I hope they're happy they've alienated a vibrant and growing audience.