In 2008 Yahoo made a bid to buy Facebook for
$1 billion. At the time, Facebook's lead
investor, Accel Partners, told a 22-year-old
Zuckerberg to accept the offer and to sell to
Yahoo for $1 billion. But the well known
investor Mark Andreessen urged Zuckerberg
not to sell.
"Every single person involved in Facebook
wanted Mark to take the Yahoo  offer,"
Andreessen recalled. "The psychological
pressure they put on this 22-year-old was
intense. Mark and I really bonded in that
period, because I told him, 'Don't sell, don't
sell, don't sell!'"
After Zuckerberg turned down the bid from
Yahoo, he explained his decision that he
couldn't imagine himself doing anything else
than building a social network.
So to answer you question: If Zuckerberg
wouldn't have founded the social network
Facebook he would probably have founded
another one. Something like FaceMagzine or
PeopleNet or HeadList or...