This is not accurate. SDS was going great guns through 1970 at least. It had led major student strikes at Harvard, Columbia, San Francisco and CCNY. When a new semester started in those years, the first SDS meeting would draw hundreds of students.
SDS's problem was that the New Leftists were babes in the woods politically, while the PLP was highly disciplined and bent on a split course. In the next radicalization in the US, there will be similar formations. Marxists have to work in such organizations to keep them nonsectarian and sane, not to try to take them over and turn them into wings of the "vanguard" party. If Marxists had participated in SDS in this way in the late 60s, it is entirely possible that an organization of 30 to 40,000 leftists would exist in the US. Instead, the sectarian madness of the 1960s destroyed this most promising group.
Louis Proyect
(http://www.panix.com/~lnp3/marxism.html)