How to represent the interests of a Facebook user
The interests are categorical data and may be modeled as binary variables (a user either likes them or he does not). You can subsume little-used categories under broader categories. For example, a user who likes a little-known horror movie can simply be marked as liking horror movies. You can even subsume such items under multiple categories if it belongs to several.

For what you can do with the data see A Review on Data Clustering Algorithms for Mixed Data