According to TimesOnline, Microsoft wants a 5% share in Facebook and is willing to pay $500 million (US) to get it. Counter offers from other big name Internet companies are expected too.
Last year Facebook was offered $1 billion from Yahoo!, but Facebook founder,turned them down because he wants to keep Facebook all to himself. Who can blame him, it's been very successful.
On the other hand, advertising is always needed to keep a big site like Facebook running. That's why they are accepting advertising offers from Microsoft. Will it turn into anything else? That remains to be seen.