Although men and women follow a similar number of Twitter users, men have 15% more followers than women. Men also have more reciprocated relationships, in which two users follow each other. This “follower split” suggests that women are driven less by followers than men, or have more stringent thresholds for reciprocating relationships. This is intriguing, especially given that females hold a slight majority on Twitter: we found that men comprise 45% of Twitter users, while women represent 55%. To get this figure, we cross-referenced users’ “real names” against a database of 40,000 strongly gendered names.

Even more interesting is who follows whom. We found that an average man is almost twice more likely to follow another man than a woman. Similarly, an average woman is 25% more likely to follow a man than a woman. Finally, an average man is 40% more likely to be followed by another man than by a woman. These results cannot be explained by different tweeting activity – both men and women tweet at the same rate.

And this makes Twitter different than other social networks:

These results are stunning given what previous research has found in the context of online social networks. On a typical online social network, most of the activity is focused around women – men follow content produced by women they do and do not know, and women follow content produced by women they knowi

(See the article here.  via MR.)  Actually this may not be stunning at all because there is probably a very simple explanation for both observations.  Twitter is a one-way social network.  If I want to follow you I do not need your permission.  Unless you block everybody and require followers to ask permission.

Regardless of the social network, women are less willing than men to allow unsolicited followers and so they are more inclined to require permission.  So for example if I just randomly selected 100 Twitter users to follow, there will be many of those 100 whom I will be unable to follow because they require permission.  Most of those will be women.  Thus, on Twitter the ratio between the number of followers of a random woman to the number of followers of a random man will be smaller than the same ratio on, say, Facebook.  And everybody will follow more men on Twitter than on Facebook.