I don't think a comparison to EA and Blizzard is appropriate. You'd likely find a better indicator of value by comparing to something like Dave & Buster's, which sold for ~$600M last year. D&B is aimed at wholesome entertainment for adults. Zynga is targeting much of the same market with a focus on women. EA and Blizzard mostly produce games for males 16-35.
The main difference between D&B and Zynga is that Zynga is available to any one with a computer or smartphone, while D&B franchises are only built in cities of 500k+ people. That difference might create a multiplier on the order of 10. $600M * 10 = $6B. So $7B in valuation doesn't seem that ridiculous to me.
Your logic focuses on the product; mine the market. By your logic men's make-up should generate as much revenue as women's make-up. It doesn't though, and make-up exists for both men and women.
Existing game companies aren't directly comparable to Zynga precisely because they don't market to women. However, a social gaming environment like Dave & Buster's is comparable, because they target adults in their late 20s/early 30s and families, whose social agenda is driven by women.
Zynga's success comes from the fact that they tapped an underserved market: women who like to socialize through playing games.
In short: "Men spend an average of 31% more per transaction, or $15.60 versus $11.90," explains Flurry. "In fact, male spending dominates female spending across each age group by a relatively consistent margin. In the 'sweet spot' of revenue generation, 25 – 34 year olds, representing a whopping 49% of total revenue, men out-spend women by 37% per transaction."
Granted - these stats are for a different company, but I'd be surprised if they are markedly different for Zynga.
The study you linked is more recent, but such a massive swing in the other direction is probably not indicative of reality. Statistics reported by agencies are often interpreted incorrectly. The companies who run the survey are often biased and perform the tests incorrectly. I've been pitched statistics by marketers plenty of times and they usually don't know what they're talking about.
Nevertheless, your article goes to show that the spending gap between men and women in online social gaming is pretty small. My article mentions that women tend not to play console and PC games as much as men do, which supports the opinion that EA is not a good comparison point for Zynga.
I wish I could get male/female spending demographics for Dave & Buster's, but it would probably be closer to social gaming than console/PC gaming. Although you'd have a tough time teasing out money from men (husbands/boyfriends) who pay for their female companions vs. men on themselves.
Edit: Many years ago when I played Everquest some friends had a discussion of why so few women play games. One guy answered, quite aptly, "By putting scantily-clad big-breasted cartoon women and monsters bashing each other on the cover they catch the attention of men. If they wanted to bring more women in, then they should have smiling people sitting around a table and cooperating with one another."
It's probably not outrageous per se, but it's certainly on the high side.