The advancement in technology has brought a new era in terrorism where Online Social Networks (OSNs) have become a major platform of communication with wide range of usage from message channeling to propaganda and recruitment of new followers in terrorist groups. Meanwhile, during the terrorist attacks people use OSNs for information exchange, mobilizing and uniting and raising money for the victims. This paper critically analyses the specific usage of OSNs in the times of terrorisms attacks in developing countries. We crawled and used Twitter’s data during Westgate shopping mall terrorist attack in Nairobi, Kenya. We then analysed the number of tweets, geo-location of tweets, demographics of the users and whether users in developing countries tend to tweet, retweet or reply during the event of a terrorist attack. We define new metrics (reach and impression of the tweet) and present the models for calculating them. The study findings show that, users from developing countries tend to tweet more at the first and critical times of the terrorist occurrence. Moreover, large number of tweets originated from the attacked country (Kenya) with 73% from men and 23% from women where original posts had a most number of tweets followed by replies and retweets.