My friend said under WP MU structure to build a social site, one user/site needs 9 tables in DB, if 10 thousands user/sites , it will generate 90017 tables (including default 17 tables), DB will bear huge burden in its I/O, 10 thousands members just are counted a small scale site, i hope to find a social solution to build a social site of millions membership unit, was his mention right or else ?
