Last week, our custom category of 155 Social Networking websites received just over 10% of all US Internet visits. With such a large audience, marketers have flocked to websites like Facebook and Twitter to reach out to consumers during the holiday shopping season. The share of downstream traffic to the Retail 500 coming from both Facebook and Twitter increased 36% and 15%, respectively, on Thanksgiving from the previous day. Downstream traffic to retailers grew again on Black Friday and Cyber Monday while many promoted sales on fan pages and tweets.

The top retailers to receive traffic from Facebook on Thanksgiving, Black Friday and Cyber Monday were Amazon, Wal-Mart, Target, Best Buy and Toys R Us. Wal-Mart received the highest share on both Thanksgiving and Black Friday while Amazon received the highest share on Cyber Monday. One caveat is that some of the traffic is going to be natural movement between the websites with visitors going from Facebook to the retailers simply from checking Facebook often.

Last week, the share of downstream traffic from Facebook to Wal-Mart had the highest increase when compared to the same week last year.

Amazon and Wal-Mart received the highest share of downstream traffic on Thanksgiving, Black Friday and Cyber Monday. Target ranked 4th among the Retail 500 on Thanksgiving and Black Friday, then jumped to 3rd place on Cyber Monday. Unlike with Facebook, where the top 5 were consistent, QVC, Blair, CafePress.com and Kmart rounded out the top 5 on the first of the holiday milestone shopping days.

Wal-Mart also led the Retail 500 with the highest increase in the share of downstream traffic from Twitter last week. For many of the websites the downstream traffic is up from zero and highlights the first holiday season to take advantage of Twitter to communicate with shoppers.