Michael Harris from New Think Tank has posted and interesting analysis on the use (or not) of twitter by think tanks in the UK. There are also some interesting comments made to the post it self that are worth having a look.
His basic argument is that think tanks are missing an opportunity: they are only using twitter to announce events or publications but not to engage and debate with their publics (peers, audiences, staff, etc.). This is an interesting proposition but it could be argued that this Twitter is not the best tool for this and that others may be better (Nick Scott wrote about this in his posts on Digital Disruption
Also interesting is that his analysis 'proves' that the link between visibility and influence is not direct: they find rather low Twitter presences for well known and influential think tanks. And the RSA may in fact be described as a the 'least think tank' of the list at it is rarely participates in active policy influence.
Other interesting results: 71% of the top 300 staff and associates users have less that 500 followers, there are no women in the top 10 and only 7 in the top 50 (although as they point out this may reflect the composition of the industry itself), younger staff may be over-presented as a result of their more active use of Twitter, etc.