These are dark days for mobile advertising. Two years ago, the investor world could not get enough of mobile advertising. Millennial Media’s stock price almost doubled on its IPO. The lack of a mobile ad platform weighed heavily on Facebook’s post-IPO prospects. Venture investors poured money into the space.

Today, much has changed. Millennial’s stock is in the doldrums and Velti just sold off its software platform and filed for bankruptcy for the rest of its business. The new consensus seems to be that there are only three mobile ad companies left – Google, Facebook and Twitter. What should we think of all this?

First, there is technological change. The mobile ad space is advancing very rapidly. Companies are iterating through business models on what seems to be annual basis. First came ad networks and ad brokers. Now the hot thing seems to be Real-Time Bidding (RTB) platforms. And there is undoubtedly something else in the works, coming soon. But the truth is that this is not so much technological change, the database and software people we speak to all maintain that the technology underpinning these mobile ad companies is very easy to replicate. Mobile ad people we speak with all bemoan the fact that so much of their business is decidedly low-tech with revenue driven by fax and excel not big-data high-speed trading algorithms.

Instead, we are still dealing with the basics of business – salespeople, deals and customers. The big changes in the mobile ad space over the past year are largely about companies with large salesforces getting their act together to tap into the big advertising budgets. This is probably good news for Millennial, as they seem to have a strong old-school (think Mad Men) salesforce that is plugged into Madison Avenue and the big ad agencies.

However, we see this is just an intermediate stage of mobile advertising. We maintain that the true value of mobile advertising has yet to be discovered. What is the right ad unit? That is to say, what method will advertisers use to influence consumers through their mobile devices? Display ads do not seem to work that well. Neither do 30-second video segments. Is it the ‘app’ that hooks in consumers? Google would have us believe that search will be the dominant form of mobile advertising, but they are still investing heavily in a whole range of other options.

So we are big believer in mobile advertising, we are just not sure we (or anyone else) knows what that means just yet. Our sense is that there are billion dollar companies yet to emerge which will show us new ways for advertisers to interact with consumers, but how that will work exactly remains to be seen.