Halogen Network has been putting together an online advertising network over the past four years, running ads from high-end hotels, airlines and other companies on more than 50 sites as of today. The company is now announcing it raised $5 million in a second round of funding — back in 2007 — from Kohlberg Ventures and angel investors. I assume the announcement’s an attempt to garner a bit of press, and I’ll bite: It will be interesting to see how a network running ads for luxury goods plans to weather the recession.

While not disclosing revenues, the San Francisco-based ad network says it has recently grown to reach more than 10 million monthly unique users on partner sites, along with two million who see its ads in email subscriptions. But the company is not going after reach so much as wealthy niche audiences (including you, as VentureBeat has run an ad or two for the network). Halogen has apparently managed to gain  — it cites a custom Nielsen survey showing it tied with Wine Spectator for the highest percentage of affluent readers on the web.

But we all know that wealthy readers and the advertisers who want to reach them have been feeling the recession along with everyone else. To that end, Halogen has been keeping up a refreshingly plain-spoken company blog discussing the future of advertising to the well-to-do. An excerpt:

Here at Halogen, we are grappling with how to redefine luxury. Does luxury have to be exclusive? Is it about sumptuous art and living or can it be more basic? What does “luxury” mean in a world where most people have seen 40% of their wealth evaporate this past year?

The post, called “Luxury 2.0,” mentions some timely themes it may use for a self-branded lifestyles site it plans to launch in the spring; it already offers its self-branded HalogenGuides luxury travel site, pictured above. New themes will include “Innovation vs. Stagnation,” “Socially conscious consumption vs. Excessive ostentation,” “Experiential value vs. Status” and “Accessibility vs. Exclusivity.” The company wants to make sure its messaging stays tasteful considering the present economic climate.

But it’s not just a time for conservatism if you’re in the luxury ad business. “As online audience fragmentation accelerates, and advertisers demand more than simple ‘banner campaigns,’ audience networks like Halogen will experience significant growth,” said James Kohlberg, founder of Kohlberg Ventures, and chairman of Halogen Network (and also chairman of private equity firm Kohlberg & Company), in a press release about the funding. To that point: the company’s blog says it plans to expand to 25 more sites within the next month or so.