Just read an interesting post by Bruce Schneier on his idea for a taxonomy of social networking data, which he sets out as follows:

1. Service data. Service data is the data you need to give to a social networking site in order to use it. It might include your legal name, your age, and your credit card number.
2. Disclosed data. This is what you post on your own pages: blog entries, photographs, messages, comments, and so on.
3. Entrusted data. This is what you post on other people’s pages. It’s basically the same stuff as disclosed data, but the difference is that you don’t have control over the data — someone else does.
4. Incidental data. Incidental data is data the other people post about you. Again, it’s basically same stuff as disclosed data, but the difference is that 1) you don’t have control over it, and 2) you didn’t create it in the first place.
5. Behavioural data. This is data that the site collects about your habits by recording what you do and who you do it with.

Interesting discussion in the comments (so yes, you should go and actually read the original post!) about whether Bruce’s taxonomy works - for example whether 2-4 are all Disclosed data subsets - 2. Disclosed data (controlled), 3. Disclosed data (entrusted), 4. Disclosed data (incidental); whether there should be an additional facet covering Crosslinked Data; and whether the term ‘ “Incidental Data” is obvious as a label or should be called something else.