Twitter Inc has bought 900 patents and signed a cross-licensing agreement with IBM, making peace with Big Blue and bulking up on its intellectual property portfolio as it takes on larger rivals Google and Facebook. The agreement announced on Friday comes after International Business Machines Corp accused Twitter in November – on the eve of its high-profile initial public offering – of infringing three of its patents. At the time, it underscored how few patents the six-year-old social media company possessed in relation to more established rivals. A cross-licensing agreement will help safeguard Twitter against similar claims in the future.