GS is not an endorsement
All GS care about is that think they will be able to turn a profit from their investment. ie. they will be able to find someone that will buy for more than they paid.
It could mean that GS thinks FB has a rock solid business plan and that FB "value" is based on fundamental asset values.
It could equally mean that GS thinks that there are bigger suckers (or greedier people) out there that will buy from them.