"Facebook shares are falling because Wall Street doesn’t know what to make of Mark Zuckerberg’s News Feed changes"

"Facebook is rethinking the way it works, and that will have repercussions for users, advertisers and publishers that use the network.

The overhaul, which will first show up in Facebook’s News Feed, will be a “major change,” said Mark Zuckerberg, who says users may end up spending less time on the site.

Wall Street believes him. And for now, Wall Street thinks this is not good news: Facebook shares dropped 5 percent overnight.

Why? Here’s one theory: “There is too much uncertainty relating to the economic impact of Facebook’s pending News Feed changes for us to be comfortable,” Stifel analyst Scott Devitt said in a note this morning when he lowered his rating on the stock from a “buy” to a “hold” — even though he thinks, “Facebook is doing the right thing for the long-term sustainability of the platform.”