Any financial analyst knows you need to look at year-on-year ("YoY") growth rates for monthly data to eliminate the impact of seasonality. For example, retail sales for a store might fall off sharply in January compared to the preceding December, simply because lots of people were shopping in December for Christmas presents.

So instead of comparing June 2010 to May 2010, you compare May 2010 vs. May 2009 to June 2010 vs. June 2009. So you could say something like, "Facebook registrations were up 30% YoY in May 2010, but were up only 2% YoY in June 2010. Actually, at this point, YoY growth rates might even be negative (i.e., Facebook may have signed up more users in June 2009 than it did in June 2010).

Month-on-month (MoM) comparisons can be interesting, but you need to put them in context to determine if seasonality is affecting the results.

At any rate, the number of new registrations, by definition, says absolutely nothing about whether users are getting burned out on Facebook. You'd need a metric like average time spent on the site per user per month to say something about that.