Adara has recently announced an integrated partnership with Twitter that allows travel brands to retarget users with specific behaviors on the social platform.
This partnership is a continued evolution of the advertising features on the platform, one that follows Facebook's Exchange insofar as it brings social inventory into the more-traditional display advertising fold with targeted ads according to specific behaviors.
For example, an airline could target a Twitter user who had recently purchased an airplane ticket with a message promoted an upgrade for the flight. This particular example becomes even more interesting, as advertisers can also take advantage of granular information like loyalty status.
Melissa Stein, Adara's VP of Product:

One of our initial campaigns was for an airline, Delta, who wanted to send a personalized tweet promoting an upgrade to people who have booked a ticket within 48 hours [of their flight]. We know that this person just booked as this person is a client of ours. We can now find them on Twitter and target.
This partnership has been in the works for many months, and has clearly been a part of Twitter's ongoing push to deliver conversion tracking to advertisers in order to allow more direct proof of ROI.
Stein shares more about the partnership:

Several months ago we started working with Twitter as part of three companies in their beta program as they wanted to start taking data from outside of Twitter and connect that to personalize tweets and accounts on Twitter. What's unique about Adara is our data relationships - unlike other companies, we're getting direct information from travel suppliers. Whether booking information, loyalty status, we have this real data. That information is then synced with Twitter's cookies, and we get a set of matched users. We also then are able to see who's new, according to who visited Twitter via the desktop.
In terms of results, Adara claims that they are seeing greater-than-average engagement numbers - with conversions being tracked in terms of engagement, the rate is around .3 to .4% for their current campaigns. Stein continues:

One of these campaigns, which was pre-purchase, saw conversions over 2% which is exceedingly high. So we're seeing really good conversions. Another value is incremental reach - are you reaching people with your message that you're not seeing in other places?
For some of the initial campaigns, we were seeing 60-70% incremental reach. So that means that these people were all new, and we weren't reaching them with any other display inventory. We know this because we were also running a simultaneous display campaign so we could test and do an overlap analysis.
In regards to mobile, Adara understands the limitations here, as their current setup requires initial usage of a desktop computer in order to receive information surrounding a particular cookie. Once that cookie has been connected to a Twitter handle, the company can then continue targeting on mobile.
The company is also considering how using the device ID of the phone as a similar cookie-style tracking datapoint; however, once a user wipes cookies on the desktop or clears their device ID, this process resets itself.
This is also a bit of a change, as anonymous cookie data is now tied to an actual Twitter handle - thus rendering it actually identifiable as far as who this person is and what their online behavior entails. The same applies to Facebook, and other name-based social networks entering the world of targeting advertising, where cookies become far less anonymous once they're tied to an actual identity and not anonymized completely.
Nonetheless, users are increasingly willing to accept less privacy in exchange for more transparent, relevant advertising that matches their personality, needs, wants, and general behaviors.
As far as the future of retargeting via social, there's still a lot of work done in order to really make it a marketer's paradise. Adara, when considering their own Facebook integration, sees the potential growth in this particular means of social advertising should social networks allow two-way sharing of social information .

Right now, we don't get a lot of these social signals. So they're not sending back tweets, retweets, or the virality effect. Social engagement has still stayed in the realm of the social network. The signal is one way.
We get the information from the travel companies, and we use that information to find consumers on social. The social intent is not passed back the other way, but I think that's going to happen more and more. I can understand that the social networks want to maintain some control, but this can be mutually beneficial.
It will be interesting to see in the future if the social signals can come back to outside companies in order to enhance the product as well. For example, if we know that someone constantly shares travel content online, that would give us a great signal.
NB: Magnifying glass image courtesy Shutterstock.