Social media has played a central role in several of the most successful reputation management campaigns of the last few years.

In a recent Forbes.com article Scott Davis describes how Toyota repaired its reputation following the bout of recalls that sent it plummeting in 2009 and 2010. The automaker employed a series of innovative social media strategies to engage with its customers on a personal level.

One of Toyota’s strategies was a series of Digg Dialogg Q&A sessions, which allowed the company to respond directly and candidly to customers and critics. Mashable’s Todd Wasserman offers a fascinating account of those sessions, which “gave Toyota the appearance of achieving social media branding nirvana: Transparency.”

That approach clearly paid off, as Harris Interactive’s latest Reputational Quotient survey, published in February, ranks Toyota as the company with the most-improved reputation.

Harris Interactive’s survey reveals that few companies have seen their reputations improve lately. Joining Toyota in that small group is BP. Having embraced social media in the wake of the Deepwater Horizon disaster, the energy giant recently ranked third in The Group’s FTSE 100 social media index.

Simply building a presence in social media isn’t enough. AT&T, for example, implemented an extensive social media customer service strategy aimed at rehabilitating its tarnished reputation. But that move, along with other improvements, hasn’t been able to counter larger issues of trust and transparency, such as its failed merger with T-Mobile and throttling of some unlimited data plans.

The factors that separate an effective strategy from a failed one can be difficult to generalize. The best strategies seem to be tailor-made for a company, its customers, and the problems it is facing. Davis’ article sums it up nicely: “Reputation is something that is both fragile yet resilient. The ability to spring back when it’s damaged takes a keen understanding of the factors and attitudes that shape a reputation to begin with.”