You do grant MN a licence to use your copy but that's not the same as owning your work or helping themselves to the copyright. Here's the relevant clause:
By submitting User Content to us, simultaneously with such posting you automatically grant to us a worldwide, fully-paid, royalty-free, perpetual, irrevocable, non-exclusive, fully sublicensable, and transferable right and license to use, record, sell, lease, reproduce, distribute, create derivative works based upon (including, without limitation, translations), publicly display, publicly perform, transmit, publish and otherwise exploit the User Content (in whole or in part) as Mumsnet, in its sole discretion, deems appropriate. We may exercise this grant in any format, media or technology now known or later developed for the full term of any copyright that may exist in such User Content.