Tumblr's servers must be quivering in fear. Lady Gaga, one of the most-followed people on Twitter, just joined Tumblr, the start-up's fashion director Rich Tong announced yesterday. Ms. Gaga tested the water by tumbling seven posts in a row, but hasn't mentioned her new blog on Twitter or added a link to her website. Maybe she's trying to give Tumblr some breathing room before her fans crash the company's servers like they did Amazon.