That's a question Hitwise's Robin Goad asked earlier this week when he showed that - in the UK - Facebook was now getting one in six page views, or 16.73% of the total. That puts it in front of Google UK (8.22%), ebay (5.39%) and YouTube (2.64%).
Robin makes his observation by pointing out that Facebook's UK growth rate is starting to slow. Just like in the US, where last month, Facebook user rates among 18-44 year olds dropped (but grew among over 45s), Facebook saw a slight dip in its UK page view share last month, which could of course be seasonal due to colleges and Universities breaking up for the year.
Facebook does of course have 500 million worldwide users, hence the question of whether it can realistically grow much more. And of course, it raises the other question of whether it will really ever be replaced (the same question constantly asked of Google in search)?
An interesting piece over on Flowtown says no, giving various reasons from the fact that a lot of people genuinely use it to manage their online lives, the applications market, and the fact that brands and businesses gravitate to it now almost as a default.