Sarah Pavis, at Buzzfeed:
Step 3. Try to download Instagram from the Google Play app. Find that it is compatible with anything at or above 2.2 (Froyo, 2 major releases behind current). If you have an older Android phone like the HTC Nexus One (not to be confused with the Samsung Galaxy Nexus or the HTC One) that has limited internal memory then odds are you many not have enough internal storage space available because the Instagram app is 16MB (which is large for an Android app). Comically sad.