Sheryl Sandberg and Marissa Mayer worked together for years at Google.

And Sheryl has championed the cause of women executives in Silicon Valley.

But now, things are about to get complicated.

1. Marissa will likely be more open to partnerships with Google than past Yahoo leaders. Not great for Facebook, which wants to keep Google isolated.

2. Yahoo's board has signaled that it's going to be a technology company first in hiring Marissa, a product-focused CEO. Also not great for Facebook, which will now face more competition not just in products but in hiring and acquiring.

So what did Sheryl write on her wall?

Congratulations to Marissa Mayer! Great news for women (and men!) across Silicon Valley. All of us at Facebook look forward to working with you in your new role.

We also want to thank Ross Levinsohn for his leadership in reestablishing an important relationship for Facebook and promoting innovation in the broader tech community.

Yeah, things are about to get a lot more complicated, given that Marissa is good friends with the Twitter leaders, too.