A major milestone in the history of the social web was passed yesterday when LinkedIn became the first big social network to go public. The company’s shares listed at $45 dollars and ended the day over $90, proving that getting millions of people to give you their personal data for free really does work as a business strategy.

At least, it does for some. Consider the two big beasts of social networking, Facebook and Twitter. While the former’s fortunes will likely be boosted by today’s news, the latter looks somewhat adrift.

Many analysts reported that today’s events make Facebook more likely to take itself and the accounts of 600 million people onto public markets. One told Canada’s Financial Post:

“I wouldn’t be surprised to see [Facebook] go on a bit of an accelerated path once they see the warm reception that LinkedIn has received.”

That story and others claim that Facebook will try to emulate LinkedIn next year. Facebook’s COO today told Reuters that doing so sooner or later is “inevitable”. The New York Times reported today that sales of private stakes in the company value it at $80bn.

Similarities between Facebook and LinkedIn suggest it has a good chance of realizing that on public markets. Despite much talk about the risks of a new internet bubble, most commentators agreed on one thing: LinkedIn is valuable because of the combination of it making real money, and still signing up new members.

“LinkedIn is growing revenue—the company just reported that Q1 revenue in 2011 was up 110 percent to $93.9 million,” reported the Washington Post.

LinkedIn has more than 100 million users and makes money from advertising, premium subscriptions and charging money to headhunters to use its network. Facebook can make similar claims: its user base is still growing fast and ad revenues are soaring, as TR reported in our most recent issue:

“Facebook ads hauled in nearly $2 billion in revenues last year…This year, revenues are on track to reach $4 billion.”

Unfortunately, that formula doesn’t apply to the other poster child of social media, Twitter. The site has a reported 300 million accounts, with a 750,000 new ones appearing every day. But making money from them is proving difficult. Earlier this week investors.com reported that Twitter’s nascent advertising features hadn’t impressed early clients:

“Online ad agencies see mixed results from early tests of ads on Twitter, a sign the microblog service could have a tough time competing with Facebook, Google and others in the multibillion-dollar online ad market.”

Twitter’s stripped-down but powerful feature set is at the heart of its success, but also seems to be limiting its money making potential. A recent feature that made ads more visible to Twitter users on the iPhone - dubbed the #dickbar - had to be rolled back after user outcry. LinkedIn’s ripples mean little for Twitter unless is can find ways to eke revenues as well as 140 character messages from its users.