We present c2d Spitzer IRAC observations of the Lupus I, III, and IV dark clouds and discuss them in combination with optical, near-infrared, and c2d MIPS data. With the Spitzer data, the new sample contains 159 stars, 4 times larger than the previous one. It is dominated by low- and very low mass stars, and it is complete down to M ≈ 0.1 M_⊙. We find 30%-40% binaries with separations between 100 and 2000 AU with no apparent effect on the disk properties of the members. A large majority of the objects are Class II or III objects, with only 20 (12%) Class I or flat-spectrum sources. The disk sample is complete down to "debris"-like systems in stars as small as M ≈ 0.2 M_⊙ and includes substellar objects with larger IR excesses. The disk fraction in Lupus is 70%-80%, consistent with an age of 1-2 Myr. However, the young population contains 20% optically thick accretion disks and 40% relatively less flared disks. A growing variety of inner disk structures is found for larger inner disk clearings for equal disk masses. Lupus III is the most centrally populated and rich, followed by Lupus I with a filamentary structure and by Lupus IV, where a very high density core with little star formation activity has been found. We estimate star formation rates in Lupus of 2-10 M_⊙ Myr^-1 and star formation efficiencies of a few percent, apparently correlated with the associated cloud masses.