The only social networks studied that didn't have more women than men in the 18-24 year old group were venerable old LinkedIn (where incidentally the 25-34 age group was tops) and a site called Perfspot.

Other highlights:

• Women ages 14-24 dominate activity on social networks and have more friends than men of the same ages.
• Men ages 35+ are more active and have more friends than women of the same ages.
• The average social network user has 2-25 friends.
• There are a disproportionately high number of 69 year olds across various social networks. (my guess is that it's the most popular 'fake age')

Rapleaf states that it "analyzed people who are on at least one social network and in which there exists age information on these individuals." The study was done 10 June '08 and approx 90% of the 49m respondents were from the US. Here is the full data, courtesy of Rapleaf:

Gender and Age Analysis of Social Networking Users: Social Network Sites

Social Network	Gender	Age Groups
		14-17	18-24	25-34	35-44	45-54	55-64	65+
Bebo	Women	1,207,833	1,373,653	735,666	197,297	84,106	33,693	12,950
	Men	569,510	802,474	488,944	162,689	63,119	27,058	10,775
	Unspecified	15,532	15,865	3,977	1,197	406	101	22
Blackplanet	Women	120,981	346,629	164,383	47,500	13,660	3,361	1,814
	Men	55,856	212,479	140,077	52,483	16,099	4,309	1,781
	Unspecified	3,114	9,027	4,870	2,152	843	240	29
Classmates	Women	142,757	599,895	724,253	240,863	117,584	41,578	10,152
	Men	62,885	278,908	435,742	211,079	100,527	41,874	12,527
	Unspecified	2,532	9,355	9,363	5,346	2,811	1,323	407
Facebook	Women	784,214	1,685,029	767,619	184,057	72,743	21,441	10,270
	Men	357,017	977,753	609,655	177,662	62,033	22,024	8,545
	Unspecified	29,495	82,958	47,769	13,403	4,595	1,549	405
Flickr	Women	87,720	303,941	363,220	139,090	60,707	19,871	5,113
	Men	44,170	235,015	398,061	205,631	89,587	33,994	8,998
	Unspecified	5,163	23,806	25,753	10,982	4,825	1,926	524
Flixster	Women	2,221,835	3,258,823	1,841,543	658,189	297,477	93,020	27,204
	Men	1,146,532	2,583,675	1,840,241	671,368	271,350	90,236	26,387
	Unspecified	439,005	936,040	728,514	309,983	132,917	56,386	16,674
Friendster	Women	341,386	1,165,896	890,380	210,887	61,603	18,889	8,364
	Men	225,834	975,965	904,600	279,728	85,178	27,573	11,975
	Unspecified	5,856	21,879	19,569	3,998	597	141	82
Hi5	Women	1,382,273	3,078,898	1,475,824	412,150	175,018	52,250	16,800
	Men	724,153	2,610,316	1,927,297	612,917	231,727	76,374	22,358
	Unspecified	374,960	833,937	453,346	143,102	55,487	16,872	3,556
LinkedIn	Women	3,697	39,594	178,550	69,197	24,368	7,726	1,355
	Men	4,618	42,642	222,431	124,759	45,310	16,083	3,379
	Unspecified	610	7,905	27,858	13,456	5,264	2,005	402
Multiply	Women	115,117	352,590	194,957	51,304	19,488	5,829	2,270
	Men	55,054	261,803	194,818	63,000	25,247	8,846	3,042
	Unspecified	184	536	389	112	44	17	0
Myspace	Women	5,158,453	7,091,214	3,800,542	1,252,287	542,694	167,087	71,531
	Men	3,365,442	5,226,788	3,238,471	1,209,510	475,566	167,101	66,852
	Unspecified	3,147	4,726	2,540	1,137	548	251	67
MyYearbook	Women	637,510	578,018	239,646	91,832	37,531	10,871	5,345
	Men	280,131	292,263	127,999	55,766	23,582	7,503	3,145
	Unspecified	20,524	20,980	9,300	4,507	1,837	729	232
Perfspot	Women	84,840	158,003	91,200	31,375	14,192	4,033	1,077
	Men	66,643	317,958	260,641	86,707	29,974	9,494	2,790
	Unspecified	30	181	264	95	36	6	0
Tickle	Women	743,111	1,491,975	887,369	318,578	151,490	44,742	12,876
	Men	309,858	939,737	739,932	268,239	118,031	41,130	12,042
	Unspecified	70,562	177,297	100,108	34,037	14,204	5,048	1,235
ALL SOCIAL NETWORKS	Women	6,322,060	9,651,584	5,683,422	1,929,328	857,965	279,684	97,858
	Men	4,050,429	7,546,654	5,543,729	2,113,597	873,135	323,251	108,731
	Unspecified	682,756	1,456,780	1,045,381	428,357	181,913	72,196	20,240

Gender and Age Analysis of Social Networking Users: Number of Friends

Number of Friends	Gender	Age Groups
		14-17	18-24	25-34	35-44	45-54	55-64	65+
Only 1	Women	494,290	1,082,078	774,348	332,421	187,938	69,763	17,661
	Men	299,845	860,299	842,264	409,445	204,222	84,208	23,848
	Unspecified	163,006	376,327	371,830	185,357	89,406	39,707	11,944
2-25	Women	1,479,294	2,480,716	1,712,634	717,988	341,775	105,332	25,928
	Men	840,014	2,171,495	1,919,974	822,654	338,124	110,903	29,058
	Unspecified	294,199	606,504	441,939	174,253	66,304	23,410	5,837
26-50	Women	270,902	637,285	412,133	140,068	47,173	9,074	3,261
	Men	184,205	577,287	410,987	120,648	33,534	8,889	3,632
	Unspecified	27,318	81,351	42,756	9,768	2,804	653	134
51-100	Women	299,873	818,744	453,323	105,268	27,640	5,518	3,509
	Men	239,245	720,874	409,077	88,815	21,004	5,826	3,871
	Unspecified	24,307	74,446	35,330	5,584	1,359	260	72
101-500	Women	750,850	1,656,855	505,277	81,626	22,222	7,517	9,867
	Men	573,135	1,447,347	525,489	93,797	22,862	8,063	9,089
	Unspecified	90,430	176,024	45,515	4,453	871	189	104
501-1,000	Women	90,868	151,497	28,281	6,187	2,159	921
	Men	58,242	142,456	45,275	11,604	3,242	1,113	1,345
	Unspecified	22,511	24,417	2,611	267	65	14	22
1,001-10,000	Women	26,950	42,770	11,855	3,355	1,111	399	709
	Men	20,824	35,819	21,446	7,558	2,202	623	765
	Unspecified	5,276	12,981	2,879	1,217	352	147	613
10,000+	Women	148	505	328	73	24	5	14
	Men	90	484	628	249	66	26	23
	Unspecified	11	17	10	0	0	0	0
At Least 1 Friend	Women	3,386,077	6,827,175	3,885,996	1,383,558	628,907	198,125	61,800
	Men	2,194,686	5,919,758	4,153,066	1,546,963	622,988	219,002	70,843
	Unspecified	621,771	1,339,069	939,981	379,682	160,809	64,233	18,113

You can also download the full dataset as a spreadsheet here.