Profitable, yes. Vulnerability to competitive threats looking to undercut its position in the market, also yes. Zynga is a media company, and as such it's competitive advantage lies in its ability to use the FB graph and continue to pump out games. Will people get tired the constant barrage of new titles in the same way Facebook's users are starting to tire its core product? I would speculate yes, but that remains to be seen.
If there are lots of successful, profitable tech companies that are not IPOing, then something else is wrong.