Fannage is a phenomenon where, in wikis, things relating to pop culture get more attention than more mundane topics, even if the more mundane topics are more relevant to real life. According to wiki law, this shouldn't be a problem, as it encourages a larger number of people to edit. Likewise the opposite isn't bad either, if a handful of people are hopefully interested in a single topic to make good entries even if hedge trimmers complain about superfluous articles being longer. On one hand, you don't have to look at the fan-tacular articles. On the other hand, Wikipedia's $12,000 funds drives every couple of years seem to be mostly going towards rewriting the Star Wars Expanded Universe in encyclopedia form.
The danger occurs when topics with high amounts of Fannage lead to a larger pool of unskilled editors. More importantly, things that attract fandoms can start getting filled with weasel words, fanon, and other mess in an attempt to fit into the format.
This is probably best represented in the often ridiculously detailed television episode synopses on Wikipedia, whose owners tolerate it mostly through fan stubbornness of editors despite being frowned upon by official rules. That they can exist for shows with negative continuity doesn't stop people treating it like they aren't.
It overlaps heavily with what Wikipedia calls "fan cruft" and is generally avoided by reminding editors that there are other wikis. Wikia, in particular, seems to exist as a network of wikis for bodies of fiction so grumpy Wikipedia editors have somewhere to evict fannage to.