Facebook has just filed a sixth amendment to its S-1 filing to IPO in order to provide more transparency about how the shift of its user base from the web to mobile is causing it to show fewer ads per user, which could hurt revenue in the long term. Facebook also granted about $796 million in restricted stock units to employees less than a week ago. I've excerpted the significant changes and embedded the whole S-1 below. Specifically, Facebook is warning investors that daily active user count is rising faster than the number of ads the site is showing, which it predicts will lead to a lower average revenue per user.As we noted when Facebook originally filed, it hasn't proven its ability to monetize mobile yet. It now has Sponsored Stories ads running in the mobile news feed, but it can't show nearly as many ads in this format as it does on the web, where it often shows four to seven ads per page, though less prominently in the sidebar. read more..