Sourceforge boasts 169,282 registered projects. The actual number of active projects may be as low as 15,000. This is still an impressive number, but it may not be enough to stave off the Google threat.
Just two years after Google kicked off project hosting on its Google Code site, Google is reporting that it now hosts over 80,000 projects. Given how new it is (and how infrequently Sourceforge prunes its projects, if at all), it may well be that Google now has more active projects hosted on its Google Code site than Sourceforge.
The real question, of course, is how important or relevant these projects are. I've not heard of many (any?) high-profile open-source projects moving to Google Code, though there certainly are some making the move. Regardless, it may well be difficult to compete with Google's hosting and search expertise over the long haul.
Will the world notice a diminished Sourceforge? I think so, but maybe I'm just nostalgic.