HTC Trophy is Solid...
The HTC Trophy has been my first smartphone and primary smartphone for some time. While it is not a "high-end phone". It is a solid phone in performance. The Windows Phone OS is designed perfectly for your needs of heavy email, texting, and social site integration.
I have about six email accounts tied to my HTC Trophy and can browse through them and get updates with only a few glances and clicks. In my opinion it is the smoothest and best option out there for email functionality. The UI for texting and social updates is stream-lined really well, in a matter of seconds through one posting you can update Facebook, Twitter, Linked-In and more.
Perhaps I'm biased, but I have nothing but praise for Windows Phone and the HTC Trophy. I have had my phone for a little over a year now and it has NEVER froze on me. Once in a blue moon a hiccup or few seconds delay, but it is the first phone I can truthfully say to have never frozen up on me.
If you can find the HTC Trophy for semi-cheap I would have no hesitation in going for it. Just be aware in October-December Microsoft is announcing Windows Phone 8 with likely indication of expanding Windows Phone handsets within Verizon. So if you're wanting the HTC Trophy for a sort of Interim phone I would say just do it, and it will serve you well. I recently got the HTC Trophy for my sister, new for $50 and she also had nothing but praise for it (and she is picky). So be aware new stuff is coming, but if you need something now, the HTC Trophy is a solid choice. Good luck with your decision!
*(Edit: Regarding Instagram, I do not believe it is currently available, however I have come across many comparable apps to Instagram in the Windows Phone Marketplace so you should be able to find something that will suit your needs.)