Open up Google and type in everything you know about the person in keyword format; for example, "sarah los angeles writer tech." Even if you only know their first name, keywords related to their job, marital status, location and school will likely bring up social networks or other identifiable results.If no social networks pop up in your initial Google search, you may need to go into the social networks themselves.If you know one or two of these things about your subject, you can narrow down your search and then browse through the photo results.If your subject has no social media presence, try to find their friends and family members; it's possible they're hiding their account behind a fake name.
I would get an Email saying so and so wants to meet you, then go into user name search and see who they were talking about.
If you have no idea who their friends and family members are, and you know their full name, use a free people search like Intelius to look up relatives..then hunt down those relatives.
Individual data points don't mean anything unless they can be connected to other data points to make up a person's online presence.
People recycle usernames, passwords, and social media profile pictures.
Grab their profile pic from their Facebook or Twitter account and plug it into a reverse image lookup such as Tin Eye.
Facebook is the most popular social network, and it has the most robust search engine, so you should probably start there.