This paper examines an Israeli professional community on Twitter, which connects people working or studying the field of Information Science. The participants were 31 individual and organizational users and 175 structural connections established among them during March 2007-November 2011. The data was extracted using NodeXL application and analyzed in terms of user's investments (i.e., participation by tweeting and following others) and gratification mechanisms (i.e., influence on the audience: the degree of centrality into the community network, the number of followers and tweets marked as favorites). The results show that (1) The evolution of the community over time was in a bell curve, in concordance with the Diffusion of Innovations Theory (Rogers, 2003). (2) Concerning the rate of active participation, five of the most active individual community members (16.1%) shared 54% of the community tweets. Thus, the head of this "long tail" distribution is relatively small, indicating a moderate level of participation inequality in the community. (3) Regarding user investment and gratification mechanisms, participation by tweeting and following others is highly related to the visible form of influencing community audiencethe number of followers, moderately related to the hidden influence -location of the participant in the community network measured by PageRank, and unrelated to the content quality -the percentage of user's tweets marked as favorites by others.Keywords: professional communities of practice on Twitter, structural connections on Twitter, influence on the audience, active participation, network analysis by NodeXL.