There are stories around why the limitation exists at all. The official reason is that Facebook wants to make sure that people only add “real” friends to their account, and the restriction is on the high end of the number of friends that any one person could reasonable have. The unofficial (and actual) reason: scaling problems made this necessary. I've heard this directly from Facebook employees, as have others.
But those scaling issues have been resolved, we hear from our source, and the cap will soon be lifted.
Facebook says that “less than 1,000″ users have 5,000 friends today. There are around 70 million active Facebook users, so the number of users who are affected is around one thousandth of a percent. But a disproportionate percentage of bloggers and press are at the limit, so the issue tends to get a lot more attention than it otherwise would.
High profile blogger Robert Scoble is among the 1,000 Facebook users who've hit the cap, and has complained about the restriction in the past.
Facebook says that the “Pages” feature is meant for people and brands that want to have a lot more “friends” than are allowed via normal accounts. An example is Barack Obama's Facebook page, which currently shows 820,000 supporters.
But for many people, being a friend is much different than being a fan, and the level of interaction allowed is also significantly different. And the new Friends List feature, which allows users to classify and group friends, makes organization easier anyway.