Honestly, this isn't a new trick. Several orchestras are already doing this. Other orchestras could learn to leverage this technique to build new audiences via social media
Both Facebook and Twitter are about engagement. However, what most orchestras currently do with their social media is to talk 'at' their audience and not with them. By changing the way they post and tweet, by devoting a bit more time for their social media campaigns, orchestras could find a wealth of new patrons for their concert halls. They will certainly expand their existing fan base. The key to this technique is re-posting/sharing what classical music fans are already posting.
It is possible to post on other people's timeline as a person, but not as a page. When someone posts something to an orchestra's page, it doesn't show up in the main feed unless the administrator(s) actually shares the post. If orchestras would encourage their fans and musicians to start posting on their page, anything they find of interest - share what's happening in the community, what's happening with other organizations, or just anything they think the orchestra should be interested in - this will do two things.
1. Give the orchestra an idea as to what their fans are talking about beyond what the orchestra is already posting.
2. Give administrators the chance to post this on their own feed.
Not only will this increase the amount of posts an orchestra is putting on Facebook, it will be more inter-active with their fans. Their fans will get excited about the prospect of being involved with the orchestra. Fans that get their posts shared, will go further by spreading the new and other posts orchestras do even more. Other fans will be encouraged to share as they will feel there is a dialog happening. Fans will feel empowers and part of the orchestra. Fans that are part of the orchestra will not only spread the word, they will be more likely to purchase tickets when the time comes.
If orchestras can get their musicians to post their thoughts on the orchestra page, sharing these thoughts, share who the musicians are as people. Again, fans will feel a greater sense of connection. The more connected the fans, the more likely they are to attend concerts.
The same basic process is true with Twitter, but you come about it in a different way. With Twitter, orchestra administrators need to seek out interesting people to re-tweet. Orchestras should re-tweet more orchestra news, not just their own. Find local business you can support by re-tweeting what they're talking about. The flip side is they will be more likely to support your tweets and your concerts in the end. Get your fans talking and mentioning you in their tweets. When they do, re-tweet them. Their tweets may not be directly related to your orchestra, but if orchestras show they are more than just flogging tweets about their performances, followers will more likely engage with what you're doing in all aspects.
This does require orchestras to devote more time to social media than they are currently doing. It does mean they will need to have one person managing both Facebook and Twitter, to ensure the message on both platforms is consistent, so fans on one can be converted to followers on the other. It will also be important to filter what comes in so the orchestra doesn't just became a spam machine. Yes, this takes time, and at a time when orchestras are tightening their belts it's a hard pill to swallow - particularly considering their is no direct correlation between social media and ticket sales. However, as the current audience dwindles, orchestras need to find new audiences. Social media is a perfect medium for this.