The Cambridge marketing firm HubSpot, which helps small and mid-size businesses reach prospective customers using social media, is announcing Monday that it has raised an additional $35 million.

The investors include Altimeter Capital, Cross Creek Capital, and, according to two people with knowledge of the deal, the Boston mutual fund giant Fidelity Investments. The new commitments will bring HubSpot’s total fund-raising to about $100 million since it was founded in 2006 by two alumni of the Massachusetts Institute of Technology’s Sloan School of Management.