The US Federal government FERS plan is not in lieu of Social Security, it is a supplement to it. It's relatively small, about 1% of high 3 year salary average per year of service.
CSRS was a very different pension plan, and those under CSRS did not contribute to nor draw social security. CSRS was phased out in the 80's, I don't recall exactly the year. All new federal employees since then contribute to social security and are under the FERS system.