Try the "Export your data" experiment
Jeremy's post about independent publication got me thinking... While not everyone's about to abandon the silos and self-publish everything, it should at least prompt you to consider where your data is living - and whether you can even get a copy of it.
Even if you don't suddenly close all your accounts and ship out, you should consider periodically backing up your data. Think about the hours you've put into services like Twitter, Facebook, LinkedIn, even Spotify playlists... if they disappear tomorrow, would you be annoyed? Is there any info in there you'd miss?
If yes, shouldn't you back it up?
So if you find yourself nursing a hangover and quietly mooching about on your computer on New Year's Day, perhaps you should make it Data Liberation Day. Go through services you use and see which ones let you get a copy of your data; and make a copy.
You'll quickly realise not all backups and exports are easy to use and the quality of the backup certainly varies. Twitter gives you a searchable archive; but Last.fm gives you JSON. Facebook gives you an archive but leaves out a lot of detail - photos and links from wall posts aren't included; and your friends list is very literally a text-only list of names.
If you do go through the data backup process you may decide to change your habits a little: for example, I post very very little original content into Facebook. Instead I let other services post into it, either directly or using IFTTT recipes.
You may not feel the need to change anything at all... but at least you'll have a snapshot of your data as it was at the turn of the year.
Some links to get you started...