I am currently in the research and planning phase for a mobile app, I have never done a mobile app before so please bare with me if I missed some obvious things. The final app will allow users to find other people with similar interests in their vicinity and allow them to meet up, similar to Foursquare.

My question for this thread is, what are the different possibilities of sending data from one phone to another using only the internet? (more specifically, sending the updated location and maybe some chat messages)

From what I have found so far I can use some form of P2P network or use web services to relay data between the phones using a central server. Are there any other possibilities? I will probably (still very early in planning phase) be using HTML5 to create a web app to be used on smartphones, are there any useful APIs I should be aware of?