Recently, Twitter has emerged as a popular platform for discovering real-time information on the Web, such as news stories and people's reaction to them. Like the Web, Twitter has become a target for link farming, where users, especially spammers, try to acquire large numbers of follower links in the social network. Acquiring followers not only increases the size of a user's direct audience, but also contributes to the perceived influence of the user, which in turn impacts the ranking of the user's tweets by search engines.In this paper, we first investigate link farming in the Twitter network and then explore mechanisms to discourage the activity. To this end, we conducted a detailed analysis of links acquired by over 40,000 spammer accounts suspended by Twitter. We find that link farming is wide spread and that a majority of spammers' links are farmed from a small fraction of Twitter users, the social capitalists, who are themselves seeking to amass social capital and links by following back anyone who follows them. Our findings shed light on the social dynamics that are at the root of the link farming problem in Twitter network and they have important implications for future designs of link spam defenses. In particular, we show that a simple user ranking scheme that penalizes users for connecting to spammers can effectively address the problem by disincentivizing users from linking with other users simply to gain influence.

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This study solves a more than two-decades-long "MoS 2 Nanotubes" synthetic enigma: the futile attempts to synthesize inorganic nanotubes (INTs) of MoS 2 via vapor−gas−solid (VGS) reaction. Among them was replication of the recently reported pure-phase synthesis of the analogous INT-WS 2 . During these years, successful syntheses of spherical nanoparticles of WS 2 and MoS 2 were demonstrated as well. All these nanostructures were obtained by VGS reaction of corresponding oxides with H 2 /H 2 S gases, at elevated temperatures (>800 °C), in a fluidized bed reactor (FBR) and a one-pot process. This success and apparent similarity between the two compounds "hid" from us the option of looking for the INT-MoS 2 reaction parameters in entirely different regimes. The main challenge in the synthesis of INT-MoS 2 via VGS was the instability of the in situ prepared suboxide nanowhiskers against over-reduction and recrystallization at high temperatures. The elucidated growth mechanism dictates separation of the reaction into five steps, as properties of the intermediate products are not consistent with a single process and require individual conditions for each step. A horizontal reactor with a porous-quartz reaction cell, which creates proper quasi-static (contrary to the FBR) conditions for the reaction involving sublimation, was imperative for the effective nanofabrication of INT-MoS 2 . These findings render a reproducible synthetic route for the production of highly crystalline pure-phase MoS 2 nanotubes via a multistep VGS process, without the assistance of a catalyst and in a scalable fashion. Being a semiconductor, flexible, and strong, INT-MoS 2 offers a platform for much research and numerous potential applications, particularly in the field of optoelectronics and reinforcement of polymer composites.

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