I "googled" your question and found this. It looks like SSA was making a verification request to establish your identity in order to establish your SSA website account (see paragraphs three and four below) and does not affect your credit score (see last paragraph).

The Social Security Administration provides an “Earnings and Benefit Estimate Statement” for all workers age 25 or older. The statement provides you with a history of your earnings and projected benefits. The SSA stopped mailing statements a few years ago, but you can obtain your Social Security Statement online at www.socialsecurity.gov.

The online statement provides eligible workers with secure and convenient access to their Social Security earnings and benefits information. The online statement provides estimates for retirement, disability and survivor’s benefits, making the statement an important financial planning tool. In addition, the portal includes links to information about other online services, such as, applications for retirement, disability, and Medicare.

When you sign up on the SSA website, they carefully verify that you are who you say you are. They ask you to provide several different types of information and to answer questions that only you should be able to answer. They may also use an identity verification service provided by Experian to help verify your identity and protect your privacy when you register to do business with them online.

When the SSA makes a verification request to establish your account, Experian may use information from your credit report to help verify your identity. As a result, you may see an entry called a "soft inquiry" on your Experian credit report. This will show an inquiry by the Social Security Administration and the date of the request.

Soft inquiries do not affect your credit score, and you do not incur any charges related to them. Soft inquiries are displayed in the version of the credit profile viewable only to consumers and are not reported to lenders. The soft inquiry should not appear on your credit report from Equifax or TransUnion, and will generally be removed from your Experian credit report after 25 months.