TLDR - Not much solid data aside from UK where numbers are stated as almost equal.
I know Facebook is not the internet, but it has a huge slice of the traffic.
These statistics from early 2009 suggest there are more female users than males using* Facebook than males. Although the data is not entirely clear it is at least based on actual Facebook data according to the article.
On average, the ratio of male to
female Facebook users is .85, meaning
that there are significantly more
Female facebook users than male ones.
This plays out with general trends in
social networking gender demographics,
which tend to show a predominantly
female base of users.
*Unfortunately this does not state clearly what they mean by usage
There are many sites out there that claim to represent a bias towards females and many sensationalist articles.
Most of these are based on "various sources".
One such example is this one from Cory Doctrow in 2007:
This has its source as eMarketer who according to themselves is based on "Data from over 4,000 Worldwide Sources". Which in itself is not a reasonable source for this site.
It seems this question came up around 2007 and there was some interest in it, but as most of it was tripe and unprovable people stopped paying attention.
One reliable source would be a national census. I am a non-American, so can't say for the USA and unfortunately for Australia the last available data is from 2000 which is now over 10 years old. A further search revealed little information from American census data that was recent.
Accoding to this report Office for National Statistics - Internet Access Quarterly Update, May 2011 from the UK Office for National statistics is says:
The numbers of male and female
Internet users were similar, at 20.51
million and 20.27 million
respectively. Men were more likely to
have used the Internet than women,
with 84.6 per cent of men having used
it compared with 79.8 per cent of
women. There were 1.45 million more
women than men who had never used the
Internet; 5.08 million women compared
with 3.63 million men.
Information about how this data is collected can be read in the linked document.