Part of the reason could merely be the lazy days of summer, which causes a drop in traffic on almost all Internet destinations. Since traffic growth was almost flat on Facebook and Twitter in both July and August, we can probably expect some healthy growth again in September. Summer hasn't been bad for everyone though, as some social sites have done very well in July and August.
Compete tells us that Facebook has had 122,220,617 unique visitors in August; a -0.37% drop compared to July, but it grew to 2,202,528,780 visits in the same period, rising 4.69%. Twitter's stats are almost the opposite: a small drop in visits (-2.99%, down to 148,651,794) but a tiny rise as far as unique visitors go, which were at 23,579,044, representing growth of 1.27%.
This numbers, while nothing to get excited about, are actually really solid. If we look at how some other social networks have been doing during the summer, we can see that it affected them strongly. MySpace has dropped to 55,599,585 unique visitors, a -6.73% drop. Bebo and hi5 have dropped -7.68% and -15.36%, respectively.
An exception to this rule is LinkedIn, which has grown to a respectable 14.2 million unique visitors in August, showing that its unique approach to social networking (let's not forget, it's one of the rare social networks with a clearly laid out business model) is still working great.
Another pleasant surprise of this summer is Digg, which has grown tremendously during this summer, beating competitors such as Reddit and Mixx into the ground.
August may have been a flat month overall, but Digg and LinkedIn have definitely surprised us with solid growth, while most others have been stagnating. It'll be interesting to see if this trend continues in the fall.