I think you need change dictionary to list of tuples and pass to DataFrame
constructor:
d = {'a':[1,2,4], 'b':[4,6,7]}
df = pd.DataFrame([(k, x) for k, v in d.items() for x in v], columns=['user','content'])
print (df)
user content
0 a 1
1 a 2
2 a 4
3 b 4
4 b 6
5 b 7
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