Crafting Tweets and Facebook posts that stimulate conversations with customers and possibly lead to sales is not an easy task. Thought needs to be put into the content, the tone and the timing. There are a few common mistakes banks fall into.

1. Ignoring social networks. Most large banks have a substantial presence on the major social sites, but there are still holdouts. "Banks are starting to realize that not being on social is like not allowing employees to be on email a decade ago," says Clara Shih, founder and CEO of social media software company Hearsay Social. "It's time to get with the program, especially if you're interested in customer experience. If that's where the clients are, you have to be there."

The cost of doing nothing in social media? "One cost is you lose business, both on the customer acquisition side and the attrition side, because clients who expect you to engage with them in these new media and see that you're not will go somewhere else," Shih says. "Clients feel annoyed when you call them, yet they still want to be educated. Social media is one of the few channels where you can stay top of mind without being intrusive."

A second cost of not being on social media is that somebody else will tell your story for you if you don't tell your own. "If you let someone else tell your story, it won't be as accurate or as good as if you do it," says Shih.

A third unintended consequence of avoiding social media is employees will go on anyway, and perhaps run the bank into compliance trouble.

"If you have good relationship managers, they're going to be client centric — they're on because their customers are on," says Shih. "If they're on without your help, there will be all sorts of rogue activity." This has led to the de-licensing of several reps who took to social networks without proper training.

In one Texas case, a financial adviser was required to serve a one-year suspension for a series of violations, including failure to inform her firm's principal that she had a Twitter account and sending more than 30 tweets about securities.

The Financial Industry Regulatory Authority found the rep's tweets to be "unbalanced," "overwhelmingly positive," and "frequently predicted an imminent price rise," and the rep failed to disclose that her and her family members held more than 100,000 shares of the securities she was tweeting about, in violation of multiple Finra conduct rules.

2. Relying too much on automated responses. In July, Bank of America got hazed in the press when an Occupy Wall Street activist wrote angry messages about the bank in chalk on the sidewalk outside a branch, and posted a tweet with a picture of himself being chased away by police. The bank's auto-responder sent out generic "we're here to help" tweets. Critics referred to the bank as a "tone deaf robot."

Such automated tweets come across as out of touch and inauthentic, Shih says. "You're using the worst of email marketing, which is spam and pre-canned messages," she says. "By bringing those practices to social media, you're degrading the whole channel. Social media is about being authentic and being a human being."

Successful users of social media in banking — and some mortgage officers, financial advisors more recently small business bankers are cultivating followings on Facebook and LinkedIn — share personal interests, videos, photos and other content.

"We're marketed to all day long," Shih points out. "An advisor who talks about 'me, me me, and buy my mutual fund, buy this stock,' is different from advisor who talks about his passion for fishing and dogs and by the way, here's a helpful article about new tax law changes."

3. Relying on keyword search. For bank relationship managers trying to attract and retain customers through social media, the typical approach is to find clues that someone is going through a major life change that would trigger a financial need.

When Hearsay Social first built its "social sales" solution, the company thought it could simply set up keyword searches for words like "baby" and "divorce." "Then we realized there were never any results," Shih says. "We analyzed this, and found that people don't say, 'I got a divorce' or 'I had a baby.' If they did, it would be easy and our idea would have worked."

People tend to use indirect language, posting messages like, "this is the hardest day of my life," or "meet Lucas, he's seven pounds, 3 ounces, isn't he cute?" The company hired a new set of engineers to build algorithms to analyze such comments and translate them into life event triggers. Whatever software they use, banks will need to be careful to sift through apparent clues to find the right indicators of opportunity.

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