Certainly not the first. I remembered there's Loopt acquiring GraffitiGeo but I'm sure there's more: http://techcrunch.com/2009/10/20/mobile-social-network-wars-...

From YC's first class, Reddit either acquired or merged with infogami (depending on who you read it from). http://news.ycombinator.com/item?id=20239

Here's another example: http://techcrunch.com/2009/06/23/posterous-acquires-fellow-y...