An article in last week's technology section of the Guardian claims Wikipedia approaches its limits and suggests that Wikipedia will stop growing because the "deletionists" won the war against the "inclusionists".
However, if you think about how an encyclopedia starting from scratch at time zero is likely to grow (given a huge and growing pool of enthusiastic contributors), it is obvious that there will be a very steep increase in the number of entries initially, as it has to "catch up" with the knowledge that was there before it started. Once all that (or at least the vaguely relevant parts of it) are included, it can slow down and only grow at the rate at which new knowledge is produced (e.g. in the scientific literature, and by current affairs). which is still significant growth rate (and probably exponential as well), but slower than the catching up phase.
So the observed slowdown is perfectly reasonable. Don't know what wars are going on behind the scenes at Wikipedia, but from the growth trend alone, there's no need to conclude that there are any.