Insurance companies in the Fortune 500 have increased their use of Twitter dramatically, according to an annual study from the Center for Marketing Research at the University of Massachusetts Dartmouth.

Insurance companies are also most likely to be on Facebook, it found.

The study revealed that the number of insurance companies in the F500 with active Twitter accounts increased to 20 in 2010, up from 13 in 2009.

Overall, some 60 percent of F500 companies now have an active Twitter account, compared with 35 percent in 2009.

Size appears to influence the decision to adopt Twitter. Half of the Twitter accounts belong to the companies in the Fortune 200, while 33 percent come from those ranked in the bottom 200.

Interestingly, the F500 demonstrate a real willingness to interact on Twitter. Some 35 percent of companies consistently responded with @replies or retweets within 72 hours, many more often.

The study also found that just over half (56 percent) of the F500 are now on Facebook. Insurance companies rank first among industry sectors with 28 having a Facebook presence.

However, the use of blogs in the F500 appears to be leveling off, as 23 percent of F500 companies have a public-facing corporate blog with a post in the past 12 months – an increase of just 1 percent on 2009.

Indeed, the number of insurers in the F500 blogging dropped to three in 2010, compared to 5 in 2009.

The study concludes:

This clearly demonstrates the growing importance of social media in the business world. These large and leading companies drive the American economy and to a large extent the world economy. Their willingness to interact more transparently via these new technologies with their stakeholders is clear.”

For its part, the I.I.I. now has seven Twitter feeds (@iiiorg @Bob_Hartwig @JeanneSalvatore @LWorters @III_Research @IIIindustryblog @InsuringFLA) with a collective following of over 3,000 users and a Facebook page with over 300 “likes”.