The three first-year halls (Barrows, Dascomb, and Kahn) house 425 people -- so more than a third of the entering class will not be in freshman-only housing. First-year halls do have a strong and rambunctious sense of community, but they're certainly not the only way to meet people. Co-ops, theme halls, and program houses (like the substance-free dorm) tend to be very friendly and welcoming to first-years; even in traditional dorms, ResEd tries to place new students together so they have a group to bond with right away.
I've spent a bit of time in Barnard, the substance-free house, and for the past few years the community there has been lovely. In terms of meeting people in college there's something to be said for choosing not the most social dorm, but the dorm where the social scene is going to be most comfortable. If your daughter knows that she'd prefer substance-free housing, she's probably more likely to find a crowd to her liking in Barnard, even if it is quieter than the first-year dorms.
There've been a few posts about substance-free housing here and on the Oberlin blogs: Oberlin Blogs | Blog Entry: "We are Gryffindor. fearless" Substance-free living space Dorms at Oberlin?