MENLO PARK, Calif. — Hundreds of janitors have been on strike since Tuesday at various Bay Area Meta (Facebook) campuses in anticipation of massive layoffs, demanding that the tech giant hold its janitorial contractor accountable to crafting a new agreement that won’t put more work on the backs of those who remain. Custodial workers at various Silicon Valley and San Francisco Meta offices have joined together this week to demand a new agreement with their employer SBM Janitorial Services — Meta’s housekeeping contractor for its sprawling campuses across the region — to reduce the number of lay…