Complex sentence: Bieber's fans on Twitter are a strong lobby, and have engaged in an effort to unseat Lady Gaga as the site's most popular account; however, fans on the service have also used it to make death threats to women connected to Bieber.
Split into simpler sentences: Bieber's fans have engaged in an effort to unseat Lady Gaga as the site's most popular account. There have also been some issues associated with Bieber on Twitter, including his fans on the service making death threats to women connected to Bieber.