To be fair with the analogy, McDonalds did not invent fast food. Drive ins and walk up fast food burger joints were everywhere before McDonalds started.
McDonalds just took efficiency to a whole new level. It used processes that pushed modern cooking equipment to its limits and invented a few new tools to fill the gaps. One store could serve and reach so many more customers every day for so much less than the competition. Combine that with repeatable best practices and an expansionist vision and you've got an empire.
By the same analogy, Zynga took that same social game model and jacked up conversion rates to the max. It pushed social networks to the limits and invented new tools to fill the gaps. It got more money from each user and had more users than anyone else. It took this formula and repeated it again and again with an expansionist vision.
Mob wars, farm town, etc looked the part, but the way those businesses operate isn't in the same league. Talk about physical presence, Zynga has multi-millions of users in 100% Zynga owned virtual real estate every day. All it needs to do is mention a new game to its millions of users and that game has an instant million user base. For a no-name competitor to do the same would require millions in marketing or a miracle.