PatientsLikeMe Inc., a Cambridge-based social networking site focused on health care, has raised $8 million of a planned $30 million equity financing, according to a filing with the U.S. Securities and Exchange Commission.

The company runs an online community for people with various life-changing diseases.

The regulatory filing does not indicate the investors in the financing, but the PatientsLikeMe website lists its investors, which include CommerceNet, Omidyar Network, Collaborative Seed and Growth Partners LLC and Invus LP.

Company president Benjamin Heywood would not comment on plans for the financing.

PatientsLikeMe was founded in 2004 by brothers Jamie and Benjamin Heywood and friend Jeff Cole. The diagnosis of Jamie and Benjamin’s brother, Stephen, with amyotrophic lateral sclerosis, or Lou Gehrig’s disease, prompted the founding of the company and launched its first online community, for people with Lou Gehrig’s disease, in March 2006. It followed with communities focused on multiple sclerosis and Parkinson’s disease in April 2007, for HIV/AIDs in November 2007 and for mood disorders such as depression and anxiety in 2008.

In February, PatientsLikeMe acquired pain management web site operator New York-based LLC, an online community where people suffering from pain can assess and track their pain levels.