Over the past six months, Facebook's monthly user base has been growing significantly, to 45.2 million unique users in the US in October 2008 according to Quantcast. This represents an all-time high. MySpace has been dropping slowly over the past two months, from a high of 72.5 million in August, to 66.8 million at the end of October.

Facebook is up, MySpace is down, at least when monthly traffic is compared. But when we look at daily user rates, we don't see that same growth curve. Facebook's daily user rates has risen 33% from around 15 million per day to 20 million per day over the past six months -- a growth rate that doesn't match the 50% rise in monthly users over the same time period.

What's it mean? Facebook has more users, but they aren't visiting as often, on average. In May 2008, 15 million people visited Facebook on a given day, out of 30 million total users -- 50% of the U.S. user base each day. Now, 20 million visit each day, out of 45.2 million total users, or only 45% of U.S. users.

MySpace, on the other hand, has held its daily users at between 40 and 45 million per day over the past six months, even when showing a modest decline from 70 to 67 million monthly users, according to the Quantcast data.

Facebook is certainly growing on MySpace in the U.S., tallying almost two-thirds the total monthly users, but the slowdown in percentage of members visiting each day is a worrying sign, if the Quantcast data is to be trusted. It could indicate that the members don't consider the site as essential as it used to be.