Python, Scipy: Building triplets using large adjacency matrix

I am using an adjacency matrix to represent a network of friends which can be visually interpreted as

Mary     0        1      1      1

Joe      1        0      1      1

Bob      1        1      0      1

Susan    1        1      1      0 

         Mary     Joe    Bob    Susan

Using this matrix, I want to compile a list of all possible friendship triangles with the condition that user 1 is friends with user 2, and user 2 is friends with user 3. For my list, it is not required that user 1 is friends with user 3.

(joe, mary, bob)
(joe, mary, susan)
(bob, mary, susan)
(bob, joe, susan)

I have a bit of code that works well with small triangles, but I need it to scale for very large sparse matrices.

from numpy import *
from scipy import *

def buildTriangles(G):
    # G is a sparse adjacency matrix
    start = time.time()
    ctr = 0
    G = G + G.T          # I do this to make sure it is symmetric
    triples = []
    for i in arange(G.shape[0] - 1):  # for each row but the last one
        J,J = G[i,:].nonzero()        # J: primary friends of user i
                                      # I do J,J because I do not care about the row values
        J = J[ J < i ]                # only computer the lower triangle to avoid repetition
        for j in J:
            K, buff = G[:,j].nonzero() # K: secondary friends of user i
            K = K[ K > i ]             # only compute below i to avoid repetition
            for k in K:
                ctr = ctr + 1
                triples.append( (i,j,k) )
    print("total number of triples: %d" % ctr)
    print("run time is %.2f" % (time.time() - start())
    return triples

I was able to run the code on a csr_matrix in approximately 21 minutes. The matrix was 1032570 x 1032570 and contained 88910 stored elements. There were a total of 2178893 triplets generated.

I need to be able to do something similar with a 1968654 x 1968654 sparse matrix with 9428596 stored elements.

I'm very new to python (little less than a month of experience) and not the greatest at linear algebra, which is why my code does not take advantage of matrices operations. Can anyone make any suggestions for improvement or let me know if my objective is even realistic?