Restaurants have a far higher barrier of entry: interior design, inventory management, supply chain, quality control, staffing, just to name a few headaches. That's not even calling into question your competence (assuming you are the chief operator).
Barriers to entry includes more then start-up costs. A restaurant can open up on a busy street and advertise in local newspapers to get a sustainable customer base. The marketing and PR costs for a social network are much much higher. They would need at least 100,000 users to A) provide value as a social network and B) make revenue from PPC ads.
